I've seen this as well. I'm not sure the cause, but for not it really doesn't matter as speed limit reports don't show up in the Waze Map editor. Waze had implementation issues so they pulled the reports out and haven't put them back yet.

If you want to submit a speed limit correct before they get it working again, you can submit a general report and enter the speed as a comment. Just make sure to tap the report button as you pass the sign so the editors can find it easily as verification of the speed.

The other option...is to join your local editing community and help get the speeds all set