Well this whole subject is fairly well regulated. And all the pilot has to do is follow the company SOP, which will insure that (s)he follows the letter of the law.
Most pilots go; if in doubt, your are not in doubt, you de-ice.
The pilots who operate more in icing conditions, will have more experience in with this subject and will on fewer occasions be in doubt, leading to maybe not de-icing an that occasion. Hence you can have two crews where one decides to deice and the other will not.
PS: when it comes to non environmental icing NEI, a low of factors comes into play, the A/C next to you might need to deice, but the A/C you are on might not, and so on