Why do people pronounce 'Inventory' differently depending on the intended context

It's mainly a British vs Americas difference.

Many (most?) Australians and Brits would say IN-ven-trie, stressing the first and third syllables. And the ven is really short, almost a vn.

Americans tend use secondary stress, so in-ven-TOR-ie.

Compare traditional pronunciations of

• Stationery
• Dictionary
• Secretary

In British English they traditionally have no stress on the vowel before rie so STAY-shun-rie DICK-shun-rie and SECK-re-trie.

The spread of American cultural hegemony plus the promotion and acceptance of regional (especially urban) culture has made modern pronunciation not so cut and dried. So pockets of North American English will pronounce them differently from the rest, the same goes for British and other English speakers.

Also I've noticed occasional difference in pronunciation by the same speaker when using inventory as a noun or a verb.

Edit: I found a reference with UK/US pronunciation examples http://dictionary.cambridge.org/pronunciation/english/inventory