Apps Written to Prefer Cell Data Network OVer Wifi iPhone

All,
I have come across a trouble recently that is rather interesting. I have noticed on my most recent billing statement I went over my data limit for the previous month. That said, I compared the times when the billing statement showed for the data usage...Interestingly enough, I was sleeping during some of those times. Example: March 27 11:30 pm is showed a rather large data usage and I went to sleep at approx 9PM that night. I have a wifi network at home and have no issues staying connected, but when my iPhone goes to sleep and I unlock it, I do notice the 3G symbol shows up first and then it recognizes the wifi. I am possibly thinking that when locked the wifi is not preferred? I read all of the articles and manuals and agree that wifi is supposed to be preferred so I an left thinking something is a touch different when the phone is in locked mode. Other than that, is it possible some apps are written to prefer the cellular network over wifi to maintain connectivity of some sort (tap zoo, etc).
For specifics, I am using an iPhone 3GS.
Thanks in advance!
Dell Dimension iPhone OS 3.1.3