Yeah, this is the confusing part for me - moving from BTC to other crypto. Also if it is based on USD price at the time of the transaction...then which one? Coinbase rates can be far different from other exchanges.
Some exchanges don't have usd. Even for those that do, it takes about 30-90 minutes to typically move ~100k in assets from one currency to another, double that if you're going between two altcoins via Bitcoin. During that time the effective usd exchange can have nontrivial swings.