I think there is a hidden "surprise demand" element. As well as increases by other suppliers that aren't enough to show up majorly in the pie charts, but still cut into the demand reaching your store.
I'm in agriculture, and I've noticed that I'll have a week's worth of lettuce stocked, go to work, and when I come it's gone. This will go on for 2 days where my factories can't keep up with my stores, then it returns to normal. The opposite also happens.
Given my self-sufficiency in terms of producing/stocking all of my own products from my starter company, I always try to have at least 7> supply of whatever is in my stores, then when I get to a truly excessive level, clear some stock in the B2B for extra quick cash. With the advent of NPC companies, this is very easy to do, if you're not fighting for every extra dollar/unit sold. Especially with high value products.
If you need quick cash, note whatever NPCs are selling the same product for on B2B and undercut them. They tend to buyout your market listings w/in 48 hours. Makes for easy bursts of growth without loans. And also reduces the amount of monitoring/micromanagement needed.
Also, quests are an easy way to get bursts of cash, depending on what they are. If you can find a good price, you can make profit by meeting the "supply" quests. If you have an agriculture or metal quest show up, feel free to message me in game. I'd be willing to sell to you near my cost so that we can both profit.
I'm Lithare Grayston on there, and my main company is Falconis, with my newer one being Falconis Metals.