Negative volumetric soil water in ERA5?

Datasets and Usage C3S - Datasets and Usage
1

When I checked the “swvl1” values in ERA5 for July 24, 2023 at 00:00 UTC, I was surprised to find 730 points dipping below zero. I saw this in both CDS and NCAR RDA nc files. Do these values have any special meaning?
The similar problem shows up in “swvl2” and “swvl3” too。

swvl1 2023-07-24-00
swvl1 2023-07-24-001038×785 85.1 KB

2

Possibly related: Why are there sometimes small negative precipitation accumulations - ecCodes GRIB FAQ - User Documentation - ECMWF Confluence Wiki

3

Thank you for your help!