Despite adding 8 destinations for 2018, Jet2 still have a much smaller base at BFS than NCL, and both Thomas Cook & TUI have a huge number of destinations on offer from NCL compared to BFS.
It's not all about the number of destinations, but also the frequency/capacity of them. Using LS as the example, BFS 3x 733, 1x 738, vs NCL 3x 733, 4x738. If they had the same frequency, then on the basis NCL has 75% extra capacity, it should be offering 75% more destinations than BFS (44 if you were wondering). But since it's only 35, it means the frequency is improved, and overall a better offering that works that bit more flexibly with customers plans.