Funny, I've seen plenty of vote tallies from Congress that show an odd number of absentee votes. Seems like they're not doing it consistently.
I would be surprised if they were. Not all votes are critical to all congresspeople. A bill on sugarbeet research might not be too critical to a senator from Rhode Island, where there are no sugarbeets (for example).
Maple syrup is not ever a likely issue for a senator from Mississippi, either.
Besides here is how it works. The positions of the Senate are known. 46 for, 54 against. Senator Smedley would vote for it, but he swallowed a golf ball and can't make it, so he calls up Senator Blowhard. Senator Blowhard is against it, so he agrees either not to vote or to vote present instead of 'nay'
Now the vote is 45 for, 53 against. If senator Blowhard abstains, it's 44 for, 54 against, 1 abstention. The bill is defeated as it would have been had Smedley not swallowed his golf ball.