Disclaimer - not familiar with the PW530 engine. But most older hydro controlled engines were controlled to N2 - but not necessarily "isochronous" (constant N2). The Hamilton Standard FCU's I'm familiar with (JT9D and JT8D) use something called a 'droop slope governor' - it's not constant speed with PLA, with a droop slope governor N2 changes with load. It's rather complicated to explain, but droop slope N2 governors will vary N2 with load - this has a rather desirable 'lapse rate' characteristics during Takeoff as the engine heats up (if you hold constant N2, as the engine heats up during TO and initial climb, N1 and thrust will drop - a droop slope will allow N2 to increase so N1 and thrust don't drop as much).
Intruder, the only CF6 hydro engine that controls to N1 is the CF6-80C2 (it controls to both N1 and N2 - N2 at near idle, N1 at higher power - far and away the most complex hydromechanical control I ever dealt with). All other CF6 engines use an N2 isochronous governor with burner pressure limiting - although the CF6-80A/A2 had an electronic "PMC" (Power Management Control) that 'trimmed' to an N1.
That's all a long winded way of saying that I suspect the PW530 uses a droop slope N2 governor - hence the change in N2 when you turned on anti-ice.