dude, you've never seen that before?
it's been around for uh... 5+ years. basically, besides the aforementioned hacks and idle timers that are available (more recent than 5 years), you can be idle but then get disconnected while being idle. when you reconnect (assuming you auto-reconnect within the allotted time period), your idle status still remains the same as it was before.
it's funny. in college, i used to get disconnected regularly and was idle for god knows how long prior to that. soon, the AIM servers had incorrect information about me (that i couldn't change -- even if i forced a signoff!) telling users i've been idle for 6+ days or so (the amount of time i was initially idle when i first got disconnected) although i hadn't been. i've seen it with friends as well. it's not unusual.
welcome to the internet!