One, carbon
composition vs. film resistors. It's not
unreasonable, it turns out, to be able to hear the difference
between them (in early stages). Voltage rating. Carbon Comps
(CC) handle HV (600 VDC) well, films not so well. Likely near
max V all sorts of microvolt-level wierdness begins in the
film jobs, plus micro-arc-overs, leakage paths, etc.
Seem to me that these are just crummy, underrated film types.
I like kiwame carbon film resistors myself. At $1.10 each they seem to
be better
quality than the 2 cent resistors. 1000 volt DV rating - 2 watts