That makes sense really. You changed about 25% +/- of your water. That means you left 75% there. So you also left 75% of the phosphates behind. So 1.5 is the mathematical result, no accounting for any rise since your last P+ test. A reading of 2.0 is about what you'd expect.
I've said it before, water changes, even significant ones (up to 25%) do little for nutrient export. It really helps with replacing elements removed.
You need a more efficient method of phosphate removal, such as Phosgaurd or a macro algae in a sump.