Sales + hotel tax in Las Vegas has just (1 March) increased from 12.5% to 13.38% to help pay for the new stadium. I imagine the SLS hasn't quite caught up with this even though the reservation system has.

However, the SPG confirmation emails do remain very confusing to us non-Americans. The deposit section always states the amount excluding sales tax will be taken, but 99% of the time, sales tax is taken at the time of deposit. I've just come to accept the anomaly.