Most of us know that platelets counts cycle as much as +/- 50 throughout the day. Here is a great reference examining variation between platelet count measurements. A little dated at this point (1951), but still very well done in showing the fundamentals (even in the 1950's, their hemocytometer was accurate to 1%).
See Tables 5-7
Table 5 - Platelet count fluctuations within same day
Table 6 - Platelet count fluctuations over consecutive days
Table 7 - Platelet count fluctuations between different measurements of same sample taken at same time
This is why the trend over time in platelet counts, as opposed to single readings, is what matters in managing MPNs.