I went through exactly this during my own prep — scored well on Practice Exams 1 and 2, then hit a noticeably different number on a paid exam and spiralled a bit. So I get why this is unsettling.
Here's what the data actually shows: the GMAT Official Prep exams (1 through 6) are all calibrated to the same difficulty pool and score scale — GMAC has confirmed this multiple times. The variation you're seeing between 685 and 595 is almost certainly not a "harder test" effect. A 90-point swing on a single attempt is well within the normal score variability range for GMAT Focus Edition, which has a higher variance than the old GMAT because of the shorter section length.
What actually explains big one-off drops: a timing issue on a specific section (one bad Data Insights section can pull the whole score down significantly), test fatigue if you sat it at an unusual time, or even just a question mix that happened to hit a weaker area. The paid exams (3–6) also have less community exposure, so students feel less familiar with question styles, which can affect confidence and pacing — but that's not the test being harder.
The more reliable signal is your consistent 650+ trend, not a single outlier. If your real exam is in a few days, I'd treat that 595 as one data point to review for error patterns (look at which section dropped), not as a prediction. Good luck — you've got the baseline to do well.