How long between two transactions? If it is long, mempool can be changed and fee will be higher or lower.
Mempool is not constant at all, it is always changing every seconds to minutes. Maybe you are using some fee estimators that are not well designed, mempool can be 50 sat/vbyte now and changed to 70 sat/vbytes in just 10 seconds and increase or decrease in just next 5 seconds.