I can explain it mathematically for you. This is the formula for calculating base stats (except HP)
Stat={[((IV+(2* base Stat)+(EVs/4)*Level)/100]+5}Xnature
When Scyther evolves, his IVs, and nature stay the same, but his base stats change. So because that number is higher, that stat goes up when he evovles into scizor. However, because Scizor has lower speed than scyther, then his speed will go down a little bit.