Auto bpm detections are just wrong. Every one I've seen hasn't been terribly great and regulalrly get it wrongs, out with the usual factor of two problem. Autobpm calculators are definately better with some audio over others, downright dirty stuff with a lot going on it can give terrible answers which are nowhere near correct.

Plus if you are sampling stuff from vinyl, if it's been recorded from a bad turntable the pitch of the track can be constantly and subtly shifting, an autobpm counter might get the bpm correct but if you try to beatmatch it anything else it becomes obvious you are getting galloping beats from somewhere.

The manual method is bay far the best and gives you the most power in Ableton, after all your BPM counter can hardly chipmunk a 4 bar section on purpose can it?