HMC8038 Isolation Problem ?

I observed a problem about the SW HMC8038. When I measure the IL from RF1 to RFC (RF2 terminated with 50 ohm), I measure 1 dB for VCTL = 1 and  41 dB for VCTL = 0 at 1.5 GHz. That means the isolation is about 40 dB but should be higher. 
Could you comment on that?