Won't connect to Internet when in WiFi range and data depleted

[smallphone]

I'm not a tech neophyte, but I am a total smartphone noob, please don't be hard on me. I have two data use related questions.

Prior to this Android phone I had a Sony Ericsson feature phone that was very economical with data use. I am a moderate email user, use Google Maps with traffic info for about an hour a day and on occasion check out Facebook.

With Android I am averaging about 4MB daily data use. It may seem laughingly small to most, but I want to keep my existing T-mobile 30MB capped plan because I am incredibly cheap. The old non-Android phone was using about 500KB a day.

I installed My Data Manager and found out that Google Maps is the biggest data hog followed by some Sony Ericsson bloatware.

But the perplexing thing is that even when I am fully within WiFi range, apps are still bleeding a little bit of connection through my T-Mobile data connection. It's small, but with my plan 200KB here and there adds up real fast.

1. Why should apps like Google Maps send and receive any traffic through the provider's data if the phone is sitting literally in the same room as the wireless router all day?
2. Now that I ran out of 30MB data, not a single app can connect to the Internet, although I am in the range of the WiFi and connected to the access point. It appears that my phone requires some traffic to go through the phone provider's connection before the bulk of traffic can connect through my ISP. My wife's Samsung phone can connect through WiFi even when her data plan is depleted. Why is my phone different, and how can I fix this?

Should I flash the phone with a newer OS? The hardware is at least three years old, I don't want to put too much weight on it with a newer OS.

 

[smallphone]

Part of the problem is solved. It must have been a fluke. I turned on the airplane mode, enabled WiFi and all apps connect just fine. Then I turned off the airplane mode and I can still connect. I don't think it was my WiFi though because I had the same issue both at home and at the office.

I would still like to understand why a small part of the data traffic is routed through the phone providers network.