When reading the results of the recent call for a general election the numbers were 298 for, 56 against. The motion failed to pass due to not reaching the 2/3 majority. It took a little digging to realise that although 298 is much greater than 2/3 of 354 (298+56) the MPs that didn't vote are also counted. This doesn't make sense to me.
I can understand why abstaining from a vote is a allowed - if an MP doesn't feel like they have enough information or has no feeling one way or another on a subject - but why does an MP not voting essentially go in the against column? Is there a underlying rationale behind this (not specifically for a general election vote, but any vote that requires calculation in this way).