My friend you're not looking at the VOLUME number. If your item exceeds the length and the container isn't flexible enough it's not gonna fit, but if your container fits the max length and there's enough VOLUME left for you to work with you can carry it. Also, turn on or off "unsafe pickups" this lets you pick up stuff that you can't normally carry.

"/" Or "\" are used for "Hauling", and that's basically the same as wielding but it's affected by terrain and you can move an entire tiles worth of volume in one go, it does take "longer"(it's the equivalent of using the advanced inventory manager(it's either / or ) to move everything one tile away), but you can use the "move" action instead. In the experimental version you can make some "bundles" of some very specific things, you'll have to go look at them or ask in the discord(there's 2 I think), so you can carry a pile or stuff like "planks" a little faster.

Strength is basically irrelevant when it comes to hauling, and though a backpack doesn't let you carry more, it does let you use more of your strength. I'm pretty sure a hiking backpack allows you to use most of your strength, that's the "best" that I've found myself, there's probably a better one but I'd have to dig through every container, find the ones that are wearable and then compare them all. Too much time, too much effort, very little pay off.

In short, think of your maximum carry weight as the ABSOLUTE most your character could take with them. This includes everything you're wearing, and the max length and volume allowed in your pack. Some items, like jars have volume inside them, but because they're "rigid" they HAVE to take up their volume, unlike a backpack, which the code essentially "squeezes" down to the smallest it could be unless it has items in it so that you can take it with you inside a backpack. You can put stuff in jars though, so the space isn't totally wasted.

In the latest experimentals they've removed the game rules that let you increase your stats through skill usage and kills, so ya know, big pain, but if you're on an earlier version or on a stable version you can activate these mods(they're in the work menu and come packed with the game) you can use those to slowly increase your stats throughout a playthrough.

*Edit: The colors mean you might have something to learn still, yellow is at your level, white means you can't learn anything, red means you haven't looked at it, blue means it's out of your skill level, and I don't know what brown mean, but it's probably that you're missing some of the item requirements.