[curiousObject]

That mistake ‘an’ is in the original blog title. I assume this was posted by the blog author

A simple spellcheck won’t detect it but grammar check might.

Edit: the reason is probably that the writer is saying “eleven hundred”. It sounds OK but looks strange in writing

[quesera]

The "an" is correct, if you read the number as "eleven hundred and fifteen".

[geerlingguy]

Yeah, it's a fun edge case of writing numbers in numeral form... it could be wrong or could be right, depending on how you read the number!