According to this uTube post, there is an insert that needs to be removed from center of at least some brands binding post cap. Once removed, the banana plug fits like expected, without unnecessary force.
Those inserts are found on AVRs and speakers that might be exported to Europe - the reason for the plugs is because their wall plugs are round and spaced just far enough apart to be able to plug them into some AVRs and speakers. It doesn't make sense to me since the male plug goes into the wall plug - it isn't electrified like the female wall plug. But like regulations here and elsewhere they don't have to make sense - they just have to be followed - or else. HK puts those plugs into all of their speakers and some AVRs.