They're not necessarily unbiased or even anonymous (in a meaningful way). The problem which is set might somehow favour applicants from certain backgrounds. The language used might be unnecessarily complex or use local idioms. The comments and variable names might betray the candidate as a non-native (but perfectly proficient) English speaker. If the candidate uses company hardware, the keyboard may be set to a different layout to what the candidate is used to.

There are lots of ways any test which (ostensibly) aims to test raw ability can be very biased indeed. A lecturer of mine once told a story about a horrible experience he had during an exam trying to whisper an explanation of the rules of Checkers to a student who was from a country where the game was rarely played - the possibility hadn't even crossed his mind.

Declaring the interview style you use as "unbiased" from the start seems like a great way to get complacent and have large amounts of bias creep in unnoticed.