When LaTeX can't place a float immediately, it places it on one of several “defer” lists. If another float of the same type comes along, and the “defer” list for that type still has something in it, the later float has to wait for everything earlier in the list.
Prior to teh 2015 release LaTeX had different lists for single-column floats, and double-column floats; this meant that single-column figures could overtake double-column figures (or vice-versa), and you observed later figures appear in the document before early ones. The same was true, of course, for tables, or for any user-defined float.
The LaTeX team recognised the problem, and provided a package
(fixltx2e) to deal with it by amalgamating
the two defer lists, so that floats don't get out of order.
(In 2015, all fixes from
fixltx2e were incorprated into the main format).
For those who are still running an older LaTeX distribution, in addition to the general (fixltx2e) package, the packages fix2col and dblfloatfix address this issue. The latter also has code to place full-width floats at `[b]` placement.