What determines what resolutions a laptop is willing to output over VGA?

What determines what resolutions a laptop is willing to output over VGA?

I'm responsible for several conference rooms and have setup 1080p
projectors and I provide both HDMI and VGA connectivity. HDMI for
DisplayPort and Mini-DisplayPort, and VGA as a fallback, universal option.
Contrary to what I expected, people seem to have much more trouble with
the HDMI than VGA, so VGA gets used a lot more than you'd think (even as
most workstation laptops made in the last 3-4 years have DisplayPort or
Mini-DisplayPort...). Also to my surprise, VGA outputs over 1080p on a
50ft cable run with very minimal degradation on certain laptops - other
laptops just don't offer 1080p as a resolution choice and top out at
1600x1200 or something else. Specific example: a ThinkPad W530 will do
1080p, a W520 won't, over VGA.
What determines what resolutions a laptop is willing to output over VGA?
I'm thinking this will come down to either a video driver that says it
supports only certain resolutions for output, or limitations of the RAMDAC
(which wouldn't be in play, at least DAC wise, on a digital output, but
WOULD on VGA, an analog output).
The basic reason for the question is that I noticed, say, a ThinkPad W520
with 1080p built in display, will output 1080p fine over DisplayPort to a
1080p projector, but will cap out at 1600x1200 (practically the same pixel
count, just a little shy) on VGA. Now, this wouldn't be surprising at all
except SOME laptops have no issue outputting 1080p over VGA, even with
lower native resolutions.
Why do I care? Well if there's some way I could enable it... for
situations where my users end up using VGA anyway, it's preferable for
display mirroring if they can output their laptop's native resolution,
which, you guessed it, is very often 1080p on 15" models.
DISCLAIMER: This is primarily a curiosity, I'm not claiming 1080p over VGA
is ideal by any means, but hey, if it works. I've seen HDMI start
artifacting more over same-length, same gauge cabling (up to 50' run in
certain rooms). If you think this is better suited to SuperUser, please
move it, but this is framed from an IT standpoint of something that
affects a real pool of users in a multiple conference room, 50+ deployed
laptop scenario.