That's just it, it doesn't.

http://www.biomedcentral.com/1472-6904/1/1

Part of the discussion says:-

"The enteric-coated formulations appear not to protect from the
aspirin-induced risk of upper gastrointestinal bleeding, irrespective of the
dose, neither for gastric nor for duodenal lesions. This observation adds up
to previous results from other investigators [4]. Assuming that the coating
of aspirin effectively spares the stomach from its topical adverse effect,
such results cast doubts on the overall contribution of the local effect in
serious bleeding lesions observed among aspirin users. Instead, our findings
support the hypothesis that the effect of aspirin, both plain and
enteric-coated, on upper gastrointestinal bleeding is mainly systemic
mediated through inhibition of platelet thromboxane A2 synthesis [19] and/or
inhibition of protective prostaglandin synthesis in the gastroduodenal
mucosa [20]. A local effect, however, cannot be ruled out as a relevant
contributor for aspirin-induced perforation, as most cases were located in
duodenum, and no difference or, perhaps, a slightly greater risk was found
with the coated formulation. A preferential prescription of enteric-coated
aspirin to patients with antecedents of gastrointestinal disorders does not
explain our results, since enteric-coated aspirin users with no such
antecedents present a risk similar to the one associated with plain
aspirin."

Lancet. 1996 Nov 23;348(9039):1413-6.
BACKGROUND: Aspirin products are known to cause irritation and injury to the
gastric mucosa. The belief that enteric-coated and buffered varieties are
less likely to occasion major upper-gastrointestinal bleeding (UGIB) than
plain aspirin was tested in data from a multicentre case-control study.
METHODS: 550 incident cases of UGIB admitted to hospital with melaena or
haematemesis and confirmed by endoscopy, and 1202 controls identified from
population census lists, were interviewed about use of aspirin and other
non-steroidal anti-inflammatory drugs (NSAIDs) during the 7 days before the
onset of bleeding (cases) or interview (controls). Relative risks of UGIB
for each type of aspirin used regularly (at least every other day) were
calculated overall, and according to dose, by multiple logistic regression,
with control for age, sex, marital status, date, education, cigarette
smoking, alcohol use, and use of NSAIDs. FINDINGS: The relative risks of
UGIB for plain, enteric-coated, and buffered aspirin at average daily doses
of 325 mg or less were 2.6, 2.7, and 3.1, respectively. At doses greater
than 325 mg, the relative risk was 5.8 for plain and 7.0 for buffered
aspirin; there were insufficient data to evaluate enteric-coated aspirin at
this dose level. There were no important differences in risk attributable to
the three aspirin forms according to bleeding site (gastric vs duodenal), or
when users of NSAIDs were excluded. INTERPRETATION: Use of low doses of
enteric-coated or buffered aspirin carries a three-fold increase in the risk
of major UGIB. The assumption that these formulations are less harmful than
plain aspirin may be mistaken.