The fee is calculated here: https://github.com/getAlby/lndhub.go/blob/c7f2d8558e7f33f020889e9de5936022554b0c1c/lib/service/user.go#L109

The problem is that we don't know the fee a priori so we are a bit conservative to increase the odds of finding a route and to make sure there is no possibility of the account going below 0. This is purely to cover the Lightning routing fees, at the moment we do not have any fees for monetization.