The key is in the directions for making the bobble:
MB (make Bobble)pick up
loop before next st
t.b.l, k1, k1 t.b.l], all into it,
turn, P3, turn, K3, turn, P1,
P2tog, turn, K2tog, slip this
last st over next st that is
You're picking up the loop before a st and working the bobble into that loop. So in order not to increase one stitch, you pass the k2tog at the finish of the bobble over the next k1 stitch. Your stitch count will remain the same.