The order differences can be seen in the example used in original accusations. Mame draws barrels and Kong at separate times and arcade does it together.
What some are holding on to is the arcade producing a swipe effect in drawing up the stage and mame doing a clean popup of the stage building. Many are missing the fact that the swipe is produced by the screen and NOT the pcb. And mame's captures are NOT produced from a screen recording but direct snapshots of the sequence from memory. This will be explained in more detail soon.
Incorrect, since the monitor doesn't dictate the order in either the arcade cab or the MAME PC.
Arcade Cab: The Video PCB interpreting the ROM data builds the level. Only the monitor displays it in the order the Video PCB has built the level. The extent of what the monitor displays is based upon the frequency of the CRT electron gun uses to shoot electrons at the phosphorus inside the tube and the actual operation of the gun which always starts at the top left then works its way down. How this interacts with the Video PCB is that the PCB is tuned specifically to the frequency of the electron gun's frequency and scan line pattern.
MAME PC CRT Monitor: MAME sends the video information out and there is no guarantee that the video hardware will match the frequency of a CRT monitor's electron gun. MAME requires a broad spectrum of frequencies as it supports tens of thousands of arcade games that each use a unique frequency set up and tuned to specific hardware. Since this is all emulated it will never be accurate to the actual hardware. This desync between the video card and the monitor combined with all CRT's starting with the upper left corner highlights this with the out of sync rendering captured on film from actual hardware. The only way to remove this desync is to code MAME to hit specific frequencies for the video card and monitors which will be a nigh impossible task as there are thousands of different video cards and monitors worldwide and function differently even on the same chipset as the manufacturers decide how the timings etc... all work.
MAME PC LCD/LED: LCD/LED monitors do not use the upper left corner start point due to the fact it is all digital. The signal to turn on the individual pixels happens far, far faster then what a CRT does and hits all the pixels needed to display their colors at the same time. This will result in behavior that does not work identically with the original hardware. I have a feeling that the behavior using an LED/LCD display on a MAME PC will be close to or identical to the MAME PC CRT. You'd have the display rendering different parts of the frame that is out of sync when compared to the original arcade hardware. This is usually a result of the MAME software sending the rendering information to the video card and the order of the output from said video card directly to the monitor using a digital signal running at a faster speed then the analog signal on top of it having more detail.
Regardless of the method of capture what is displayed is in the order of when it hits the respective input from the video out. It does not affect how the images are displayed. What does happen is the introduction of lag.
MAME's avi record functionality actually records the audio and video streams as uncompressed avi. It isn't reading the memory, but rather dumping the images and sounds of the game directly into the avi format like one records a game onto VHS or other video capture devices. The mng format does the same except the audio and video are separate files. The only difference between a VHS VCR and a video capture software is that the latter is software only while the former is hardware combined with software. The software offloads the computational side of things onto either the video card or the CPU of the PC.
An arcade cabinet's CPU is not powerful enough to handle the computational side of things which is why various recording methods do the actual heavy lifting. A computer from when the Billy Mitchell MAME recordings could go up to 3.5 Ghz which is 3,500 times faster than what the Z80 CPU on the DK Cab. It can handle the recording of game play natively through the video card.
In any case, the monitor does not produce anything. It merely reads the signals provided by the software and hardware that is further up the chain. Think of this as a locomotive. The ROM provides the power with the CPU being the engine. The monitor is the caboose. It contributes nothing to what you see other than display the information provided.