Using my hard-won extensive knowledge of why colours suck ass sometimes I put forth some theories. The gist was colour quantisation was happing somewhere, somehow and not that some balmy system integrators just liked purple.
With only 256 colours on these displays the logical answer is: RGB332.
RGB332 is well known for sucking ass, since it makes some very fundamental tradeoffs:
1. It only has two bits of depth for the blue channel
2. It is evenly distributed across the spectrum and thus-
3. It really, really sucks at grey-dients.
In their infinite wisdom (and probably through the power of being maybe slightly newer and spending more money on a display controll) HP's 620LX uploads an 18bit, 256 entry palette to its display controller.
Same 8-bits on the wire, but much more opportunity to pick your colours.