Based on my experience using the Ookla Speedtest mobile app, the Android app can't be trusted!
Have a look at the following two tests run on my mobile (OnePlus One):
The following is the router's DSL status page which I ran both tests with:
Notice the overinflated uplink result in the Speedtest app result vs. my DSL line speed.
On my fixed wireless ISP connection, it underrates the downlink:
As I don't have access to a status page showing what my Wireless ISP is limited at, I ran an FTP test right after the following tests completed:
3.4MB = 27.2Mbps, so it's obvious which result is more accurate.
These are the results form a YouTube video I created showing these tests in action. Usually the Speedtest.net app reports about 50% above my actual uplink for any connection, but in this case it was out by over 3 times the link speed! Plus catching it in a screen recording made was even more exciting as I think I only ever seen it go that high once before.