Why are people suddenly logging in underground when they logged out above ground?

Here's my list of plugins...

I use craftbukkit version 1597 (RB).... does anyone have an idea why people are logging in underground when they logged out standing above ground?

 

Yeah, happens to me when I lag on the server.

For example my server is in Canada and when I join from my home United States IP it does not happen. Although when I join from an Italian IP, it does that, due to high latency and as a result slower loading times.

Just my guess.

 

Yeah, I also tend to spawn under ice when I logged out on it while having a high latency.

 

I've actually noticed this most when logging out in an area with only 2 block high ceilings (Ie: same height as your character). It appears that when you login, Minecraft attempts to drop you from a distance slightly above the floor and since that would place your body inside of a block at that point, you get moved up until there is enough free space to spawn.

 

This is also likely to happen when logging out on top of halfstep floors.