The ERA5 monthly mean u and v wind components are calculated from the sub-daily data. However, during a month, the direction of the wind can change so that you can have both positive and negative values.
For example, consider a wind that is 10m/s westerly for half the time and 10m/s easterly for the other half. The average wind component is 0m/s, with zero speed. However, the average wind speed is 10m/s.
So for a month you could have a 'mean u' and 'mean v' which are small/close to zero - hence the wind speed calculated directly from these is too small.
In order to calculate the monthly mean wind speed from ERA5 data, users have to download the sub-daily (hourly) u and v data, calculate the wind speeds, and then average these over the months.