The Easy, Medium, and Hard labels in the Official Guide are general categorizations, not score-band mappings. There isn't an official GMAC publication that says "Medium = 600-700 level" or anything similar. The labels are bins based on overall test-taker performance, not a translation into specific scaled-score zones.
Here's what's actually going on. The GMAT is adaptive, and individual questions are calibrated using statistical parameters that describe how question difficulty relates to test-taker ability. GMAC uses those parameters to deliver questions during your test based on how you're performing. The Official Guide sorts its questions into rough bins so you can practice across a range, but within "Hard," some questions are harder than others. Within "Medium," some sit closer to Easy and some closer to Hard. The label is a useful relative signal, not a precise score equivalent.
Two more things worth noting. The live test is the GMAT Focus Edition now, scored 205 to 805, so the 600-700 framing comes from the legacy version of the test. Score thresholds and competitive ranges are calibrated to the new scale.
How should you use this practically? Treat the labels as a progression guide. If you're early in your prep, Easy and Medium questions are where you build the mechanics and develop clean habits. If you're pushing toward a high score, Hard questions are where you stress-test your understanding under more layered conditions. But don't try to map "I got this Hard question right, so I'm at the 600 level" or anything like that. That isn't how the scoring works. Your score depends on how the adaptive algorithm responds to your accuracy across the full section, not on which difficulty bin a single question came from.