The problem that the tempo of the loops created by loopers like the Digitech JamMan Stereo or the Boss RC-2 after a while vary in a certain way, in other words the looper gets out of sync and does not produce seamless loops anymore, has been discussed in many forums.
I have checked this out extensively and found out that the looper itself always reproduced the recorded loop in a constant tempo. So the reason for deviations of the tempo can be found somewhere else.
A possible inaccuracy may originate by using a drum computer or a midi click-signal while you are recording a loop. In this case the duration of the loop will be controlled by tap tempo because both devices cannot be synchronized by midi. And here you might find a possible source of error: I think that it is almost impossible to get the exact tempo by using a tap tempo. And besides the loopers do not round up or down automatically to an even bpm-value.
The drum computer is set at 120 bpm. A recorded loop has been started and stopped by footswitch and therefore its duration might correspond to a tempo at 120.2 bpm. This leads to a shifting of 0.2 bpm with every single loop and this may causes the impression that the tempo of the loop varies.
The Digitech JamMan provides the following solution: it offers the possibility to transmit the recorded loop to a personal computer. Besides the audio file every loop contains an info file. By opening this file with an editor you can see the exact bmp-value. And by feeding this value into your DAW or your drum computer you can synchronize both devices exactly.