1)
We find seven Bnei Meshelemyah, eight Bnei Oved Edom, six Bnei Shemayah, and four Bnei Chusah. This is 25 - there were only 24 places to guard!
1.
Malbim (1): Shemayah himself is omitted from the count - his sons were Roshim in place of him.
