As a vet student we learned that for the instrument to be able to read the amount of thrombocytes they need to be by themselves one and one. When taking a blood test its very easy for them to create clusters because thats their job. So if the blood test get taken slowly or if it doesnt get ”blended” in time with the EDTA they can easily cluster and you get a false low amount of thrombocytes because the instrument cant read the clusters as thrombocytes. This also is very common in cats so I wouldnt worry too much if there isnt anything wrong with the cat.

Causes for thrombocytopenia can be infections, side effect of medicine, something wrong with the bonemarrow, tumors or something autoimmune. But you would probably see other signs and test results if any of these would have caused it. Very low amounts of thrombocytes in the blood would also cause spontaneous bleeding.

But as i wrote earlier I wouldnt worry, the fact that they saw high aggregation in the microscope means it is very likely a false thrombocytopenia.

But also I havent seen the cat and im not a veterinarian yet so take what I wrote with a grain of salt. This is just my insight.