Its all random baby. Sing the u RNGesus song to make interceptors appear
Sure it's random, but there may be parameters we're unaware of.
For now, I've encountered 63 NHSS that were not NHSS 5, I've entered them all and didn't get a lone interceptor a single time.
(On the other hand, I've encountered 46 NHSS 5, and 25 of them had a lone cyclops)
Even if the change to encounter a lone interceptor in a NHSS 6+ was only 20%, and even if 33 of the 63 NHSS !=5 that I've encountered had been NHSS 3 or 4, the chance to "miss" the lone interceptor roll 30 times in a row is about 1 out of 1000...
I'm going to continue building up the figures, but I'm already pretty sure that I'm not hunting the basilisks/medusaes at the right place (which isn't really a problem for now since I'm mostly collecting materials, but at some point I'd like to fight them).
Edit: And btw, thx for TheOverWolf and space voyager for the ship suggestion.