On imdb Bill Skarsgård is only mentioned as a cast member for season 1 of Hemlock Grove NOT 2 (says 13 ep.). That is pretty weird since he is the main character of both seasons. Also, Madeline Brewer is not mentioned at all despite having a big part in s.2. Why is this?
Basically, he is not listed on IMDb because nobody has added his credits for those episodes.
A quick glance at the Season 2 listings suggests that somebody has added (some of) the guest stars listed in the episodes, but has not done a complete audit of the on-screen. Cast credits.
You can add the data if you have access tot the episodes by clicking on the Edit page button on the individual Episode pages, and adding the missing items.
Once the credits are listed in the individual episodes they are 'rolled up' to the series page, which will correct the count there.
Note that at the top of the Add portion of the form is a text link : Click here to copy existing credits from the series or other episodes.