The word agreeable is used to describe a character trait of people who "easily agree" on things, e. g. people who rarely insist in their own opinion. In the form of agreeableness, it is also used in psychology as one of the big five personality traits. People with high agreeableness are often altruistic, people with low agreeableness may be perceived as selfish.
One specific example, where I most often heard the word being used, is in the context of salaray negotiation, where agreeable people accept lower salaries more easily than disagreeable people. The words agreeable and agreeableness are used in the same context, e. g. in this article.
How does it make sense that someone who easily agrees (e. g. to a lower wage, avoiding the conflict of demanding more) is agreeable? To me the word clearly sounds like "it is easy to agree with that person", not "the person will easily agree with me".
(I recently heared another word that follows the same "logic", however I cannot remember it now. Maybe if someone can think of it or other words and point them out, the problem becomes clearer. It was another word where the person was described as [something]able in a context that suggested the person is able to do or be something.)
To me it would make more sense if the word agreeing would be used instead, because from my experience, adjectives with the ending ...able describe subjects or objects with which something can be done.