I looked through my flight log data from my phantom 4 and the time stamp interval throughout is inconsistent (2m25s.0, 2m25s.2, 2m26s.1, 2m29s.9, 2m30s.0). Why is there an inconsistant interval?
Is there a setting for the interval? (can I set it to once/sec?)
Is a measurement at a specific time an average of multiple reading before it?
I am using the DJI Flight Log Viewer from https://www.phantomhelp.com to look at flight data, is there a better way/app/website to look at it?

Is there a setting for the interval? (can I set it to once/sec?)
No .. there's no way to change it.

Is a measurement at a specific time an average of multiple reading before it?
The usual interval is 1/10th of a second.
Sometimes the Phantomhelp viewer might show bigger intervals but you could look at the spreadsheet data in those cases.

I am using the DJI Flight Log Viewer from https://www.phantomhelp.com to look at flight data, is there a better way/app/website to look at it?
It's the best.