I transferred 28 to the person, but it could had been 27 as well. It is not a 2-credit difference! Each credit is worth 0.50 so two credit will equal $1.
From the screenshot Bob (BobBarr) attached to his comment (taken from page 4 of your training), it appears they do. In your case, it seems they deducted 30 credits from your account, but the recipient (KPollen) only received 28.