Querying the URL http://api.tvmaze.com/singlesearch/shows?q=The+League always shows the information from http://www.tvmaze.com/shows/22040/justice-league
I can't give you an exact answer, but the reason this happens is that the singlesearch endpoint returns the highest scoring result.
Normally extra score is given to exact title matches, however some words like 'the' & conjunctions (e.g. 'and') aren't matched against, therefore it doesn't get this additional score.
Therefore it's possible due to other factors (e.g. popularity) that other results are scored higher. In this specific case, I am a little bit surprised as Justice League has a lower weight than The League (72 vs 86), so maybe @david can provide more insight here.