Tach readings on vesselview appear to be wrong

Stodge

I've been trying to chase down a tach issue with the port engine.  Based on the readings on the vesselview the engines were anywhere from 300 to 1000 RPM different when a WOT.  I had to have the throttles noticeably different to have the tachs read the same.  My mechanic just called me and said that the reading on his coil connected tach is different from the vesselview.   Any ideas if there is a setting that might affect the tach reading?  I'm not finding anything.