How can a more recent stock order be selected first when I have another older stock order open?

External question — Personal Finance Stack Exchange Author: Alexis Wilke Original post: https://money.stackexchange.com/questions/166782 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I placed a limit order a while back to buy 93 shares of a stock. One day, I got 10 shares so I immediately placed them for sale. It sold on that day. So I placed another buy for 10 shares at the same price as the original buy order. So obviously, the 93 shares order (now 83) was placed before the 10 shares order. Yet, when the purchase went through, the 10 share order was selected first. Note that both orders used the exact same limit price. Would that just be a trick from my broker (Charles Schwab) or is that something normal on the market? To make it clearer, here are the events: Sep 29 -- place buy limit order of 93 shares Oct 1 -- got 10 shares purchased, Sep 29 order shows another 83 still for sale Oct 1 -- place sell limit order of the 10 shares Oct 1 -- got 10 shares sold!!! :) Oct 1 -- place a new buy limit order of 10 shares at the exact same price as on Sep 29 Oct 3 -- got 10 shares repurchased, Charles Schwab shows order from Oct 1 as filled instead of order from Sep 29 decreased by another 10 shares I really thought that instead of what happened on Oct 3rd, the Sep 29th order should have 73 remaining shares for purchase, and my order from Oct 1 would still be opened. I'm wondering whether on the books, it is as expected and only my account shows this funky discrepancy. Do you have an explanation?