For people who are shy or introverted they will constantly be saying one thing and doing another. They will expect you to take them at their word and ignore their actions. The easiest and most common example is they will make plans and then just not show up. For what ever reason they just flake without warning and won't tell you unless you call them.
For their friendship they will occasionally hang out when it is convenient for them. To them that little bit of attention and being with you meant so much, but to someone who is out going or an extrovert. It means very little stacked against what you have promised to do in the past and the times that you made plans but never followed through with them. Being there one time out of 10 does not a reliable freind make.
Someone who is shy or introverted will place a high value on your friendship and your advice and company. However they won't show it with their actions and sometimes if you ask them they won't even tell you how much you really mean to them. Often an extrovert to an introvert is the great friend that helps them be somewhat social. Because they were social last week this week they can rationalize just lazying around instead of hanging out. Cause they were social last week and they need to recover their social energy.
They have your back, unless they really need to defend you against someone. Yup, you're arguing with another person and you look to them for help and they are either hiding or behind you stuck in their own mind. No help what so ever. Of course later they will give their side of it and defend you online. To their friends and family.
We have an unspoken and unshown bond with them. We put in the time and effort to friend them, gain their trust, and open them up. Now they act pretty much the same as before they are just less shy. So now what? We just invite them and even when they rsvp we don't expect them to follow through.
So far this has been my experience with shy/introverted people. Comment below if you want me to do another mytake but on other extroverts.