It may seem that these DIYers are choosing to parallel them to reduce inductance (especially without air space between them) although it may also be a temperature concern. If these do in fact produce simple distortions then the apparent results could possibly be mistaken for an inductance issue. Personally, I'd prefer to keep them within their ratings. Carbons are not very forgiving and tend to burn.
Power isn't a big issue because it are often short peaks and you can choose it a bit overrated to get higher power and even less inductance.