The 4th import also added 1997 entries (most towards the end)

Hello Anthony Rudd,

Please note that in a Translation Memory if you have a segment deleted or changed during the time, the translation unit number has not been reallocated (only if you made a re-index for the translation memory) and when you make an Import the numbering is automatically re-indexed. This can be a reason of the different number of translations units.

My advice is to re-index the old TM and see if you have the same number of translations units like in the new created one.

Please let me know if works.

Wish you a great day.

BR,

Ana-Maria Matefi

Thanks. Reindexing helped, but it is unfortunately that this is not performed automatically with the import. 
A reimport into the reindexed TM still resulted in 2421 entries (out of 164186).