I think those snow depth charts have been widely discredited haven't they ?
They seem to count even a flake of sleet as full-on snow, so if there's 1mm of rain (with the odd flake of sleet) it'll appear as 1cm of accumulated snow.
IMBY, which is shown as having exclusively rain and sleet, has a cover of 2cm of snow as a result. It doesn't add up, but for areas where just snow is forecast it's still a relatively useful tool.
(You can see the precip type by changing to "significant weather" on the list of parameters on weather.us).