Per NWS: Some of the variables that come into play include...
Depth of the warm layer from the surface into the snow producing cloud. The warmer it is (closer to freezing), the lower the ratio will be.
Amount of ice in the snow producing cloud. If there is more super cooled water droplets in the cloud, snow ratios will be lower. If there is a higher amount of ice crystals, snow ratios will be higher.
If its windy, snowflakes can fracture, losing their "lacy" structure and leading to lower accumulations (lower snow ratios).
Deep cold, in general, promotes higher snow ratios.