Let me answer @elegant's question. I don't have enough reputation to answer there, so let me answer here:
In the answer by @Pie Bie, Pie Bie says that people used the horizontal line symbol to mean they had already double-checked a value, so they needed to find a different symbol. If you're in accounting you really don't want it to look like you had checked a value when in fact you hadn't.
@elegent asked why people used the "-" to mean "double checking" if many other symbols were avaiable.
We do a similar thing in Brazil, and in fact I thought this might be common. Given how we move the pen to create this horizontal line, I'd say it evolved from a check mark (✓). Let me explain the hypothesis.
To start doing check marks, you touch the pen in the paper, do a small downwards movement and then an upwards movement, both being kind of diagonal. But double checking in accounting is a LOT of repetitive work; people get tired and lose fine motor control, and so they start doing less and less of the downward movement. But the less they go downwards, the less they can go upwards, because it would increasingly start looking like a forward slash ("/"), which is already a division sign and would also create confusion.
And so now they're pressing the pen against the paper, making what's essentially a dot, with only the "intention" of doing the initial downwards movement. I mean, they didn't just start consciously doing a clear, objective, straight horizontal movement: after all, they don't want it to look like a minus sign.
I made a drawing of how that would look like. (Pardon the quality, but I just wanted to emulate a tired person writing.)
So your check marks start becoming worse, this actually happened to me when grading tests. Of course, I didn't "come up" with it, I based it off seeing my teachers in middle/high school who did the same, and it was common enough it happened automatically and I just assumed my students in uni would understand (I mean, I wouldn't let it get this bad if I thought it would be unrecognizable). Heh, I do remember one teacher in high school doing it in a way that was impossible to distinguish from the ／, which they used for wrong.
Anyway, I suppose that to someone doing accounting on their own, the degradation of the check mark didn't matter, because they were only marking to themselves anyway. But if someone else is checking down the road, or, maybe your office grows and it's multiple people, it starts causing confusion, especially because most people would never let their check mark degrade so much.
Well, I hope I helped, even though the question is kind of old I doubt anyone would stumble over this explanation.