The team has investigated this issue and confirmed that it's not affecting any core functionality. They will continue to work on finding a resolution to this number count disparity in due course. We hope it doesn't cause too much hassle in the meantime.
10:42 PM PST - Feb 26th・See in your timezone
You may see incorrect member counts for channels in the channel browser. Sorry for the eyesore, we're getting this sorted.
10:08 PM PST - Feb 26th・See in your timezone