Speed test is irrelevant in real world situations. If you go in and manually set the server location you will get a different speed on each one. I did 3 different servers and got 3 different speeds. It all depends on Google server and the current load. Over wifi you could have been connected to server A(slower download speeds) over cell you could have been connected to server B(faster download speeds).
Also signal is also depending on the number and type of towers around you. Pay attention the server that is automatically connected to when you do a network speed test. It isn't the same one your wifi connected to automatically.
Seeing as how most people don't download games via cell network. That server is less congested which leads to a nice download speed, the server that you connected to, to download the game could very well possibly be closer to your location.
Things that determine download speeds are distance from server, how many other devices are downloading from that server. Also the time of day can determine you speeds as well.