Hey Quick, I think I may have figured the answer, but someone more knowledgeable may have to confirm.
From the HD recruiting guide:
When a player signs, that team’s attention points will be reduced based on the number of Opening Allocation of Attention points still available divided by the # of openings.
So you had 40 AP based on openings (60 total minus the 20 base). You don't get extra resources from the ones you recruited and then cut. So when you signed McEuen you then had three openings left. 40 original AP divided by three openings is rounded to 13. 60 minus 13... bam, 47.