Where did you get your figures from?

Here is a spreadsheet of general election results, with the relevant Northern Irish parties picked out and summed (I hope I didn't miss any). Data is from here which may not be the most authoritative source, but it had the results in a structured format, and I don't see any reason it would be wrong.

What may be causing confusion is that until 1974 the Ulster Unionists took the Conservative whip. That means for some purposes they could be counted as Conservatives, something that's done in this House of Commons briefing paper (see the note on Ulster Unionists).

What you may notice from those figures is that there is a considerable rise in MPs for NI parties but in 1983. I don't off-hand know why that is - I think it may be Labour voters switching to SDLP. I will look into this more later if I have time.