It is my understanding that flight time and time in service should be the same.
If that is what you think, then you don't "understand" the subject at all. Try re-reading FAR Part 1 - both FT and TIS are defined there, the definitions are essentially as I already mentioned above.
The formal definition of FT says that it starts when the aircraft starts to move under its own power and goes until its parks or shuts down ("comes to rest") again. The odd thing about FT is that you don't have to actually fly in order to log FT; you just have to taxi out with the intention
of flying; if you happen to discover something during your run-up that you don't like and decide to taxi back and shut down, you can still log all of that time as Flight Time.
Time In Service (TIS) on the other hand is only the time that you are actually flying (go figure!) as the formal definition is the time from when the aircraft leaves the surface of the earth until it touches down again at the next point of landing. In other words, there is no TIS until you actually do fly - and only while you are flying.
Theoretically, at a large airport for example and per my "bad run-up" example above, you could easily log 0.5 hour of Flight Time for the pilot for the time he spent taxiing out to the run-up area, actually running up the aircraft and finding something wrong, and then taxiing back to the ramp again - but during all of that the aircraft itself would have accumulated 0 hours of Time in Service.
They are completely different things, recorded for completely different reasons, and there is nothing about either of them that implies in any way that they should be the same - 'cuz they ain't!