Actually, the Vanadium content in 7Cr17MoV makes it a closer steel to 440B or 440C than 440A. The carbon content more closely resembles that of 440A, but the added Vanadium apparently increases the heat treating and/or hardness capabilities.
STAINLESS Steels -
http://www.agrussell.com/Steel_Guide/a/73/
Carbon (C) Manganese (Mn) Chromium (Cr) Nickel (Ni) Vanadium (V) Molybdenum (Mo) Tungsten (W) Cobalt (Co) Typical Hardness
1.4116 0.45-.050 0.40 14.50-14.80 — 0.10 0.60 — — 55-57
13C26 0.65 0.65 13.00 — — — — — 58-60
14-4CrMo 1.05 0.50 14.00 — — 4.00 — — 60-62
154CM 1.05 0.5 14.0 — — 4.0 — — 58-62
19C27 0.95 0.70 13.50 — — — — — 61-62
20CV 1.90 .30 20.00 — 4.00 1.00 .60 — 60
3Cr13 0.32 1.00 12.00-14.00 — — — — — 54-56
4034 0.42-0.5 1.00 12.50-14.50 — — — — — 54-55
420J2 0.15 1.0 12.00-14.00 — — — — — 49-53
420HC 0.40-0.50 0.8 12.00-14.00 — 0.18 0.6 — — 56-58
425 Modified 0.40-0.54 0.50 13.50-15.00 — 0.10 0.60-1.00 — — 57-59
440XH 1.60 .50 16.00 0.35 0.45 0.80 — — 60-62
440A 0.65-0.75 1.0 16.00-18.00 — — 0.75 — — 55-57
440B 0.75-0.95 1.00 16.00-18.00 — — 0.75 — — 57-59
440C 0.95-1.20 1.0 16.00-18.00 — — 0.75 — — 57-59
7Cr17 0.60-0.75 ≤1.00 16.00-18.00 ≤0.60 — ≤0.75 — — 54-56
8Cr13MoV 0.80 0.40 13.00 0.20 0.10 0.15 — — 58-59
As you can see, the makeup of regular 7Cr17 without the Vanadium is virtually identical to that of the 440A, except for the addition of some nickel in the 7Cr17; and its typical hardness rating of 54-56 is also quite similar to the typical 55-57 hardness rating for 440A. But there are quite a few knives out there now made of 7Cr17MoV with HRC of 58, like the Tekut knives getting issued to some police departments and military. A HRC of 58 would make the 7Cr17MoV more similar to 440B, 440C, or even 8Cr13MoV, which all typically rate 57-59. Since the Vanadium content is the only component differentiating regular 7Cr17 from 7Cr17MoV, I can only conclude it must be responsible for the rather significant increase in hardness properties.
Edit: A little chemical and elemental homework on Vanadium confirms that even a very small amount of Vanadium strengthens steel alloys considerably.