Why is it "noticeable" and not "noticable"?

Here's something weird. I made a typo today in an essay and got minus points for writing "noticable" rather than "noticeable".

I thought you drop the vowel at the end of the verb when you add -able.

Solution 1:

Words do not always drop their final vowel when a suffix is added to them. Usually, I think, it is based on pronunciation.

'Pleasurable' can be easily pronounced.

'Noticable' looks like it ought to be pronounced 'notickable'.