Why do the text alternatives for Informative images examples have colons?

[corinedwards]

The text alternatives for Informative images examples use colons at the end of each string. There is no explanation as to why this punctuation is used.

There is a mention of being consistent with visual presentation, ie.

Consistent with the visual presentation, the text alternatives “Telephone:” and “Fax:” are used to identify the device associated with each number.

..however it is not explicit if the colons are a part of the recommendation. Is a short description AND a colon significant to making that ALT text consistent with the visual presentation?

[brianelton]

Since the image and the text is presented in the same paragraph, some screen readers will read both without any pause, which could affect understanding. The colon will provide a pause between the text alternative and the number.
With any situation like this (whether to use the colon or not), it's best to test how it is announced by a screen reader to ensure it is understandable.