Good question, hitman911946
As I understand it, 'precise range to target' should be the range as measured using an active sonar, i.e. by pinging the target, whereas 'estimate range to target' is the estimation made by the sonarman, based on target's sound as heard on hydrophone.
The discrepancy between different range estimations seems logical to me, though a difference of 1,000 or so meters for consecutie estimations (done within few seconds one from the other), makes me to think that our sonarman is probably affected by a premature form of Alzheimer's disease.
On the other hand, I've never used the precise range calculation, for 3 reasons:
1. I am not sure that type VII U-boats were eqipped with active sonars;
2. using an active sonar should involve the risk of giving up our presence, but I am unsure that this fact was modelled in game;
3. last but not least, as noted by you the 'precise range to target' gives ranges
consistently wrong 
Welcome aboard!
