Because "1x the performance of a Surface Pro 4" didn't get past the guys in marketing.
Well 1.05x but yeah. Truth hurts!
The new surface pro is not a huge update from SP4 in terms of processing power. SP4 is still in the game, just without the extra pen sensitivity and battery life. That's my guess.
it's also now getting towards time to retire my SP3
To get a higher number, no doubt. However, it may be excusable given that many buyers may be coming from SP3.