An easier way to think about it it instead of using "high white" use "strange patterns" for the definers.

The stranger or more unique "special" the pattern the more likely the rats (or its parents) were bred specifically for that pattern...at the risk of everything else (this risk is usually health quality).

These strange patterned rats are also typically higher in white in non-typical locations (head, neck, feet, etc.)

Hope this helps.

Remember there is no guarantee that a low white rat won't carry mega or something else and no guarantee an odd patteren/ high white one will...the risk is just higher.