Theres a few things to take into account.
There are a good number of seats where the LibDems are the natural opposition (though Labour's strong position has lessened them for this election)
Tactical voting will be a thing in this election like never before, there are already a number of campaigns underway and some of them have people like Carol Vorderman leading the charge for them
The LibDems are strong in the South and South West (especially) and will mop up a good number of seats in that region.
I was conflicted in the poll above as my current thought is they'll get between 45 - 55 seats, I went for 50+ in the end but it is a very fine line
I'm also of the opinion that their vote share will be higher than their current (still pre GE announcement) place in the polls and as we get closer to the poll their numbers will rise with tactical voting to the 12-15% range. Up to this point people haven't really looked at where they live and how they'll vote if they vote tactically and that rather than policy will be where the LibDems rise.
You can also add in that in the local councils they won control of a good number of councils that weren't exactly expected, they are on something of a rise which isn't showing yet (IMO obviously)