CSS Flex-Growth Rules

Despite what you might read online, setting all your flex-items inside a flex-container to a flex-growth of 1 will not necessarily give all the items the same size. Per the spec:

the flex grow factor, which determines how much the flex item will grow relative to the rest of the flex items in the flex container when positive free space is distributed

So, if you have no free space because your flex-items are full, it's entirely possible that your flex-items have different sizes despite a flex-growth of 1 on each.