This recently just happened to me. It didnt "seem" to happen until I tried to mess around and add a dual monitor. I was like fine. I hate 60 hz as it is but Ive gotten used to it over the past 2 yrs.
I noticed it said 59hz. I may just have a sharp eye but to me it looked slightly "watered". Went to change it to 1920x1080 HD 60 hz and the highest it went was 59.
I said eff it and chose SD 1920x1080 HD 60hz. It took the setting and asked did I want to keep it. After saying yes it reverted back to the 59hz mode. I was like wtf. I didnt want unsintall drivers clean as nothing is technically "broken". Ill do a clean install when the next nvidia driver is released because I dont prefer custom modes.
Found this article.Followed instructions I had to manually add the 1 in the 60.0001.It still sets that back to 60.0000 but so far it looks like it used to so Ill take it. I have no clue as to what caused this cause I know for a fact it was on 60. I did notice that my HDCP status is now unlocked and it wasnt before, maybe if I can re-lock it, it would "fix" it. For now ill try this.
**EDIT....Dxdiag reports the monitor is running at 59 hz