They're not, actually. The shields are 5x3 squares (you can see when they're on the floor). The backpack is 5x5, so they should fit. There's code in the game specifically to prevent them from fitting (no idea why - it's perfectly possible to carry another shield on your back, if you can handle its weight).
I could understand maybe 5% chance of being "blocked"; even that isn't realistic for a wooden fence vs. a bullet, but I suppose it could alter the trajectory of the bullet slightly and make it miss (and since the game doesn't support that, "blocking" would be a substitute). But a 24% chance to stop a bullet fired by a rifle, that isn't even travelling through it (it's travelling above it) ? That's a bug. It might be a design bug (not a typo in the code), but it's still a bug, and should be treated as such. You can clearly see the bullet travelling above the fence and disappearing. If a piece of thin air above a crate blocked bullets in CoD or some other shooter, no one would call it "a design decision", they'd call it a bug.
It gets worse, BTW - bullets also get blocked by 10 cm tall flowerbeds. Gives "flower power" a whole new meaning.
And if shield users have never been able to climb over fences, that makes it even stranger that so many maps have areas fully enclosed by fences (and at least two of them start with all your xenonauts inside such areas).
Again, having maps that make one of the game's classes impossible to use definitely fits my definition of a bug (unless it was deliberate, in which case it feels like the map is trolling you: "haha, you brought shields, sucker!" but I doubt any serious mapper would do that, as mentioned in my first post).
The simplest solution is probably to make all fences destructible with a single melee attack, but I have no idea if this can be done globally or if it means going through every map and changing it individually.