If we fill out the statistic by saying that the TSA has a high success rate in confiscating items from people who aren't trying to hide them, but a 4% success rate in getting them from people who are trying to hide them, that works out to an overall failure rate of 96%.
We only care about trying to take these things from people who are hiding them. Inconveniencing innocent travelers is a cost of the TSA, not a benefit.
But loads of the stuff the TSA confiscates is hidden - it's probably just hidden because the TSA is really annoying, and people want to take their e.g. pistol to their destination.
No sane person would pack a handgun in carry-on baggage on purpose (the potential hassle if discovered is just too big). When stuff like that happens, it's usually either because people aren't aware or forgot that they're not supposed to do it - might sound strange, but it really happens - or it was a mistake while packing.
By hidden, I wasn't referring to guns just left in bags. The TSA calls it "artful concealment" - stuff like guns taped to steel plates (for X-ray shadow) or knives stuck in the bottom of deodorant. Maybe those people just forgot it, but that's a pretty weird thing to do at any time.
I don't see how sane plays into it - either they are sane and accidentally have a gun in their bag, or they are insane. How can you filter by sanity?
Point is, probably most of the people with guns, swords, etc are not malicious, but I haven't seen any statistics on that, only this 95% cited over and over again.
How often does that happen, though? I can't even recall last time I heard of a case of someone deliberately trying to conceal a gun or a knife in luggage.
And our best estimate of their rate of taking hidden stuff is 4%. The fact that they catch a lot of stuff doesn't mean they're getting more than 4% of stuff.
We only care about trying to take these things from people who are hiding them. Inconveniencing innocent travelers is a cost of the TSA, not a benefit.