Beta rev 50 spark plug problem

[iank]

My lads Rev 50 was due a plug change, I removed the old one NGK B5SH. A quick google indicates it should be a NGK B9ES. My issue is the thread is longer but does fit ok with hitting the piston but it also seems to rev a lot higher on tick over. Were these swapped for any reason or has previous owner just fitted an odd plug? And why would it rev a lot higher on ticker over?

Any help or info would be much appreciated.