First fo all, Spock would never say, "I thought you said that repair would only take you an hour." In fact, it's as unSpocklike as can be, as is, "Well how long will this take?"
Also, I never saw Scotty give an estimate he had to back of like this. Assuming it did happen, in-character dialog would go more like this:
SPOCK: Mr. Scott, you stated that the repair would only take one hour.
SCOTTY: Aye, sir, that's when I had assumed the—
SPOCK: Assumptions are not not based on fact, Mr. Scott. I require a realistic figure.
SCOTT: Twelve to fifteen hours, sir.
SPOCK: That is considerable margin of error. Explain.
At which point Scott better damned well have a "logical" explanation.