The fifth familiar slot costs 5M gold to unlock. I check my gold amount and it says 5M. I go to unlock the familiar slot but it says “not enough gold”… turns out it’s because I only have 4953908 gold and the number got rounded up. This is bad UX. Numbers should always be rounded down such that if it says you have X gold, you have at least X gold, not “something around X but maybe not actually enough”.
This follows standard rounding practices. Because it is greater than 50% it rounds up. If it was less then it would round down. I agree this can be confusing at times but it’s not likely something coded into the game. Rather a hard coded default line