OOC: Because it's always needed.
Surprising powers aren't considered part of a set. This doesn't mean you can take all your powers as Surprising. It's meant for an odd power. In your folks case Magic (mostly, others could apply). Or one that you need a higher Rank for. When you get the right Rank, it quits being Surprising.
When you gain a Rank you don't lose powers. Period. At rank 2 you won't go from 4 of 4 plus a bonus, you'll go to 5 of 8. with 3 more allowed. You may still get 1 or two bonuses, depending on your number of sets. But if the first two new slots (of the three) are for 2 new power sets, then you've only got one slot left to spend. You can take a 3rd power set with that last slot, but not a 4th as you have no more empty power slots.
There are exceptions to this. If you're a telekinetic with Levitate, for instance, I'd let you drop levitate for Flight 1 so you can get Flight 2, but, mainly, the point is that your powers are growing, not shrinking. You shouldn't count on losing any of them.