I have an even stranger experience actually
Just migrated to the 5X from my OnePlus Two so that I could use Google fi (day 1.5 so far) because my work phone has been using the fi service since first week in May (2016).
I have both my 6P (work phone) and my 5X (personal phone) sitting next to one another, and the signal meters are WAY different.
The 6P icon is about 75-80% and the 5X is closer to 10%. Both on google fi, phones are 1" apart from one another...
More specifically:
5X Statistics
Signal Strength: -104 dBm 1asu
Network type: CDMA - EvDo rev. A
6P Statistics
Signal Strength: -103 dBm 35asu
Network type: LTE
I will check the settings on both, but so far I haven't really messed with anything >.>