obfs4 bridge every 10-20 seconds send and receive some data on static or default browser pages

There are a few things that could affect traffic timing:

1. Javascript, especially for advertisements, could be periodically uploading/downloading data (for example reporting back to the ad service or downloading new images).

2. Tor itself sends padding cells randomly following the padding spec.

3. Obfs4 can delay traffic, but I don't believe it sends its own periodic cover traffic.

So my guess is that you're seeing Tor's padding traffic. If you disable Obfs4, you should still see this traffic.