Every Bitcoin wallet suggests a fee, and every suggestion is an estimate that can be badly wrong. The reason lies in what information is actually available.
Block space is auctioned continuously
Each block holds a limited amount of transaction data, and producers select the transactions paying the most per unit of that data. Inclusion is competitive.
The clearing price is whatever the least generous included transaction paid, which is determined entirely by who else happened to be bidding.
There is no posted rate. A wallet cannot look up the correct fee because the correct fee does not exist until the block is built.
Estimators extrapolate from the recent past
Common approaches examine fee rates in recent blocks and the distribution of transactions currently waiting, then infer what rate should clear within a target number of blocks.
This works while conditions are stable, since the near future usually resembles the recent past. Most of the time the estimate is adequate.
It fails whenever demand changes faster than the observation window. The estimator describes conditions that have already passed.
Block timing is random
Blocks arrive on average every ten minutes, but the actual interval follows a distribution with substantial spread. Gaps of half an hour occur regularly.
During a long gap, transactions accumulate and the backlog grows. A fee that would have confirmed quickly becomes insufficient through no change in demand.
Estimators cannot anticipate this because block discovery is inherently unpredictable. The randomness is a security property, not a defect.
Demand arrives in bursts
Large batch payouts from exchanges, waves of activity around new token standards on Bitcoin and market volatility all produce sudden surges.
These are not smooth. A quiet mempool can fill within minutes, and transactions broadcast just before the surge find themselves far down the queue.
Because the surge originates off-chain, no amount of chain observation predicts it. The estimator learns about it only once it has arrived.
The practical response is flexibility
Overpaying guarantees prompt confirmation and wastes money whenever the estimate was conservative. Underpaying costs nothing until the transaction is needed urgently.
Wallets supporting replacement let a user start low and increase the fee if confirmation is slow, which converts a prediction problem into an adjustable one.
That is why fee bumping features matter more than estimator accuracy. The ability to correct a wrong guess is more valuable than a better guess.