Well, to have algae, you need more than just phosphate... so perhaps you are limited in something else in your system... like carbon or iron.
Corals will adapt... so even if your phosphate is higher right now, some have adapted to it. It makes it a tad bit harsher on new additions probably, depending what environment they are coming from... but it doesn't mean instant death to your current corals.
There is also the possibility that phosphate is leaching out of the rocks currently. At your consumption rate of GFO, and the amount you say you're feeding, it might be a possibility. The skimmer being not at 100% for a bit definitely can add to that.