I believe this is because of the installed anti-icing gear on the aircraft.
I found this in one of the EASA docs relating to the Seneca V.
8.For operational reasons AFM/POH with a reduced MTOW are available. No physical changes to the aircraft other than installation of additional limitation (weight and manoeuvring speed) placards and installation of applicable AFM/POH are necessary for this MTOW reduction. With Piper Kit 766-632 (or equivalent Piper Drawing 88247-{ }) installed weights are as follows:
2009 kg (4430 lb). –Max. Ramp Weight
1999 kg (4407 lb). –Max Take-Off, Landing and Zero Fuel Weight
For aircraft S/Ns 3449479 through 3449482and 3449484 through 3449486kits in accordance with Piper Drawing88247-{ } havebeen installed during production at the factory without being specifically listed as installed options in the aircraft logbooks.
9.Operation in known icing conditions is approved if the complete optional ice protection system in accordance with the respective Piper POH/AFM-Supplement (AFM/POH Section 9) is installed and operable.