I think you're confusing two related concepts. What the Tories won at the last election is a "plurality", ie being the single largest party. A plurality isn't a mandate to govern - although technically in Britain the only mandate to govern you need is the monarch's invitation to do so.
A majority, in political terms, mean gaining more seats than ALL other parties combined. You can't just randomly choose Labour and the Libs as comparators, that'd be mathematically false: you have to include all MPs of every party, even independents. Therefore, the Tories failed to get a majority, in the most politically favourable climate for an opposition in my life time.
Plurality is exactly the term I wanted but had forgotten! Thanks.