Did someone faced problem with clarity measurements with turbinometer?

We encountered a problem with the analysis of clarity of solutions. As per Ph.Eur. reference suspension I should have 3 NTU, reference II 6 NTU, however we measured around 4.7 and 9.6 respectively.

We prepared several solution, all the same.

We bought Primary opalescent suspension with Assay on CoA of 4094 NTU, obtained standard of opalescence is 79.4 NTU (according to Ph.Eur. should be 60 NTU). Also the linear dissolution would imply 4094x15/(15+1000)=60.5. Turbidimeter ir showing almost +20NTU.

We use a 4 points calibration.

I cannot understand what more should be done?