Two things on the Cell example: (1) That wasn't in the manga. Krillin was the only one repeatedly punching/kicking Cell without effect (the anime took some pretty extreme liberties since this is the same Trunks who was later slapping Cell around, albeit briefly against a suppressed Cell. Not to mention that part about Krillin using the kiezen on cell didn't happen either). (2) For DBZ purposes, having your guard down doesn't necessarily mean not having your arms up. For example:
watch?v=MVCTdHhOCgY
The Mummy concludes Goku has no openings in his "stance" and that he's guarding himself really well.
What it really boils down to is something else entirely. You'll recall when Krillin nearly killed Vegeta on Namek. How did he do this against someone who's fighting power was roughly 50x higher at that point? Very simple.
He lowered his defenses. Now how does this works precisely is never something we're given an explanation, but we do know that guard is a conscious effort and that it can be lowered and raised unlike someone such as Superman, whose durability is entirely independent of whether he wants it or not.
I'll give you another example:
The bullets fly right through Super Buu, someone whose well beyond planetary level in terms of power and durability. We've seen him effortlessly shrug off blows from SSJ Gotenks. So why do bullets work against him? Very simple. He allowed them.
Now Toriyama never comes right out and says it, but the very likely explanation here is that guard functions based upon how much ki one is pouring into their defenses (think of shields from Star Trek). This is not to say that there's no base durability without ki being a factor, but it does explain how Krillin can nearly kill Vegeta despite being 50x weaker, how Krillin can send Recoome flying, how Gohan and Krillin can hurt Frieza despite him being in his final form, how Piccolo can send Frieza flying despite him being in his final form and even how Sorbet can mortally wound SSJSSG Goku. It's consistent and makes sense.