Tach way off
 

So I noticed while boating in choppy and very rough water the other day my tach would not go to the max rpm's it has been to. It would run max rpm's at 5,500rpm's and now it only hits 4,500rpm's and normal idle is 900rpm's and now its a bit lower around 600rpm's...so iam guessing the higher the rpm's go the more off the tach gets. The motor is deff running hard and still reving higher then what the tach is showing. I checked the basic connections on the tach and everything looks good. Its a Teleflex brand sportman series tach and I was just wondering if it could be anything else? I order a new tach for the heck of it but if anybody has had this issue please let me know. Thanks
Erik