I gave up on phosphate, I tried every test and an ICP.
For the last three years, if it's under .2 on any test, that's ok by me.
I have a ton of corals in a mixed 65g, and never had any negative result.
In addition to the above comments, how soon after feeding were the 2 tests done? What other maintenance was done? Besides corals and algae messing with phosphates, sponges and biofilms are messing with phospahtes also (along with carbon and nitrogen). Depending on variables and sensitivity, tests a few hours apart "can" give surprisingly different results. And IMO, 2 tests with just one test kit really isn't enough to go on. Here's a good article by a renowned scientist discussing this Skeptical Reef Keeping #9