How come abbreviations acquired the apostrophe when referring to plurality? [duplicate]

Possible Duplicates:
What is the correct way to pluralize an acronym?
What is the proper way to indicate possession when using an abbreviation such as Dr.?

An example:

Most DD's have good packaging skills.

How did things come to that instead of:

Most DDs have good packaging skills.

Compare that to:

Most Debian Developers have good packaging skills.

[update] This question was asked with the presumption that the first example is correct, so I was surprised to find that it isn't the case.

Solution 1:

The (IMO, mistaken) apostrophe-based usage arises out of a feeling of awkwardness in simply appending an s or es to an acronym or initialism.

Solution 2:

The first quote is still considered incorrect, although (depressingly) widely used. "DDs" is likely to be strongly favoured (or favored for our US members) here...