RPM Readings: Tach vs Smartcraft Monitor

[Hydrogasm]

I have a Cougar 22MTR /'03 300X propped with a 21 High 5 for skiing. I was out for a little zip up the lake the other day, so I thought I would let run to see what numbers I would get. I have a Gaffrig 120mph GPS speedo, Gaffrig 8K tach and a Smartcraft 1000 monitor. As I was running, I saw a top speed of 76mph, tach reading of 7,200 rpms but the monitor showed only 6,980 rpms. I assume that the monitor is taking it's readings right from the ECU, so it should be correct?? I hooked up my DDT and checked the run history and it did not show any minutes over 7,000 rpms. The fault history did not show over-limit seconds. Any opinions?? Can a tach be calibrated?? Let me know what you think.
Thanks
Gary