It's because aircraft don't stall all at once.
A stall occurs when the airflow over the upper surface of the wing separates from the entire upper surface, causing loss of lift. This airflow separation starts at the trailing edge (some separation right at the trailing edge often occurs even in normal flight). As the aircraft's angle of attack increases, the point on the wing where the airflow separates from the upper surface moves further and further forwards.
Up to a point, the lift generated by the wing continues to increase with increasing angle of attack, despite the growing separation bubble, as the wing deflects air more and more downwards. Past a certain critical angle, though, the forward edge of the area of separated flow moves forwards very rapidly until it's nearly at the leading edge, lift drops off, drag increases sharply, and the wing's stalled.
The separated airflow is highly turbulent, and, once the pocket of separated airflow spreads far enough forwards and becomes big enough, this turbulence can sometimes be felt by the pilot as buffeting or juddering as the large amounts of turbulent air flow rearwards and strike the aircraft's horizontal tail. Whether this can be felt before the aircraft actually stalls depends on the aircraft:
- Many aircraft, including (apparently) the Spitfire, have fairly-docile stall behaviour, with the separated flow producing noticeable buffet over a considerable angle-of-attack range before the wing finally stalls. These are mostly aircraft built with at least one of the following:1
- Thick wings, which cause the area of separated flow to grow slowly and gradually rather than all-at-once.
- Low-mounted horizontal tails, which allow the turbulent airflow coming off the wings in the area of separated flow to strike, and buffet, the horizontal stabilisers and elevators.
- Washout (a slight twist built into the wing going from root to tip, so that the angle of incidence - and, thus, the angle of attack - at the wing root is slightly higher than at the wingtip), which causes the airflow over the wing roots (the portion of the wing whence comes the turbulent airflow responsible for striking the horizontal tail and producing pre-stall buffet) to separate earlier than that over the wingtips,2 which provides a warning (in the form of said pre-stall buffet) earlier than would otherwise be the case.3
- Other aircraft (generally those with fairly-thin wings, high tails, and little-to-no washout, such as - rather infamously - first-generation Learjets) stall abruptly, with the area of separated flow growing almost instantly from too-small-to-noticeably-buffet-the-tail to encompassing-the-entire-upper-surface (i.e., stalled). Since there's no significant warning buffet before the actual stall, these aircraft need stickshakers to warn the pilot of an impending stall.
1: In the specific case of the Spitfire, the washout was the deciding factor (thanx @Mark and @Guy Inchbald).
2: This is especially important for aircraft with rearward-swept wings (where, in the absence of washout, the effects of spanwise airflow along the wing cause the wingtips to stall at a lower angle of attack than the wing roots, causing the aircraft to pitch up violently and be difficult to control in roll at high angles of attack), but is beneficial even on straight-winged aircraft like the Spitfire.
3: Another benefit of wing washout is that the lower angle of attack at the wingtips - where the aircraft's ailerons are mounted - enhances the ailerons' control authority (ailerons become less and less effective with increasing angle of attack), preserving roll control at high angles of attack.