Well, it's actually supposed to.
On the non VAT machines, if you select straight-party, and then proceed to change some of the selections to someone from a different party, it then automatically de-selects the "straight-party" tick-box. Kind of like how in many electronic lists (e.g. contact lists, emails, etc), where you can select or check-box each item in the list, there is often a box at the top that says "select all", and when you click that, it auto-selects everything in the list. If you then go and un-check/un-select one of the items, the "select all" box un-checks itself.