Alphabetically sorted months. But WHY?

They are probably reusing a component that happens to sort its entries alphabetically, since that is most commonly the expected behaviour. If the form is configured in a CMS, whoever built it might not even know it's happening and has entered the data properly, but it gets resorted in the presentation layer. It's also not impossible that the behaviour of the component has changed at some point and this particular case didn't have test coverage or wasn't actually part of the specification.