I managed to nail this down, the forum software uses text replacement so when certain text is posted it cross references the text against our database and from which selects which smilie is linked to the text.
Therefore if you select for example a smilie that uses the text-
: cool elsewhere (all one word, no spaces), you might see your original smilie when you cut and paste but once the 'Post reply' button is used the forum software cross references that code against our database of images and posts the relevant one.
A little tip, when posting if you type in a colon : followed by a description such as cool you should see smilie options using cool within their name (other words are available, the full list of smilies and their text replacements can be found here:
https://www.maggotdrowning.com/forums/help/smilies/