Many reasons - the first being that if you refer to the 'sender' information on West's call, the 'sender (or caller) is listed as Jeremy Bamber. If you refer to the same column on the log you believe is a record of a call from Nevil, it states quite clearly that the sender (or caller) is from CD 1990 which is West's call sign.
I've already mentioned that in my post above. And I think that can be explained by the fact that the logs are ongoing rather than a static record. I don't believe that the reference to 'CD (1990)' can override what is plainly apparent if you accept these documents at their face: there were two different callers.
West called Bonnett while still on the phone to Jeremy and passed the details of Jeremy's call over to Bonnett.
I know, but that doesn't mean that Nevill didn't earlier call Bonnett. You'll see from West's trial transcript that the call times was a controversy at trial and was the subject of West's cross-examination by Bamber's counsel.
Neither log contains the surname of Sheila as Caffell, because Jeremy couldn't remember her married name, it isn't really likely that both Jeremy and Nevil would forget her surname.
But you resolve that discrepancy yourself in your own comment. We may well ask: was Sheila still being referred to within the immediate family as a "Caffell"? My understanding is that she had split or divorced from Colin. Has it occurred to you that, whatever the formal position, Nevill just called her 'Bamber' and that might explain why Jeremy couldn't remember her married name? It may also be that the two police operators just assumed her name was Bamber, having simply been told it was Jeremy Bamber's sister and Nevill Bamber's daughter. In any case, it doesn't prove that there weren't two callers.
The log even states that the information was passed to 'CD. (West) by the son of Mr Bamber.
To be clear, it's not disputed that Jeremy Bamber spoke to PC West. That's Bamber's own statement. Bearing in mind these were evolving incident logs, it looks to me like Bonnett is recording another call, this time from PC West, who related the call from Jeremy Bamber. That does seem, to me, a reasonable interpretation of the document.
Bonnett also documents the word 'berserk' instead of .crazy' which is a word used by him again on the back of his log when describing how the dogs were baking at WHF.
Sorry but I'm not clear how that works against Bamber?