Every morning I wake up there’s the potential for something to be different in how I feel and what I can achieve.
Somedays yoga is smooth and I can get into position easily, somedays (most 
) I struggle. If I’m coming off a week of hard running and riding I stiffen up before I loosen up again. Cold weather takes me longer to feel fluid when cycling.
Power held during the fitting impacts position too. Hence endurance riding may have a shorter more upright position than racing when factoring in comfort.
Different, saddles, shoes and cleat placement across fitting sessions will also affect the position.
Climbing, descending, riding on the flat are all a compromise of one position and can be optimised to make one easier than another.
Factor all this into the bike fit session and you can see why there are differences.
Some fitters fit according to numbers from motion capture only and a given software supplier’s interpretation on what’s best for the masses. Camera placement, marker placement and calibration can affect measurements.
Some fitters work with you and have a protocol to help you find the right position through controlled experiments.
Both have their merits. Both may give different results.
HTH Scott