Can any explain what File Blend actually does?

I imported a sample into Vital and Serum. Both wavetables sounded exactly the same…with buzzing artifacts between the wavetables. Once I turned on File Blend in Vital, the sample sounded perfect.

Serum has Fade options but X-fade seems to flatten the waveform on both ends of the transition, so it actually just adds more even harmonics than needs to be there.

Anyone know what’s going on? Thanks!