It's frustrating, but I think that's mother nature for you. :/ It's not like the carb content is controlled by them, like the number of mg of a drug in a capsule. It's whatever mother nature doled out. The mushrooms one year, from one specific farm, might have different growing conditions and therefore nutrition profile from another year or another farm.

Ever notice the difference in flavours of produce? I guarantee the nutrition stats vary more than one might think. One tomato can taste watery and bland, and another will be almost ketchuppy-sweet. I bought blueberries a few weeks ago that were weird - they were incredibly tart, like a granny smith apple. They didn't even taste like blueberries.

Meats, same: you can pick up two packs of bacon, and one is obviously fattier than the other. Steaks of the same cut: some are clearly more marbled with fat than others.

The nutrition stats (ie carb counts) are just averages.

This is one of the failures of food tracking. I rarely bother. It's not that I think it's useless, but it's more for forcing portion control and measuring, helping you see errors when you're eyeballing, and for helping you see your patterns and behaviours.