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. What I am trying to tell you is that it is not that mempool is changing for long time but congestion that makes it not to decrease to the feerate set by low fee payers but yet still always changing and fluctuating with certain range when congested but not decrease below certain range that can get low fee transactions confirmed. Good fee estimators are able to give estimated fee that have less higher or lower fees than the best and cheapest fee to use with same waiting time. All estimators are not accurate 100%.
I don't understand why do you need to explain my post.
