Optional parameters in VB.Net

Here's the situation. My ASP.Net Application has 3 projects A, B and C. B contains a function (ex: testfunction) that is used by pages in and C. Now for a specific requirement for Project C I need to make changes to the test function. To do this, I added an optional parameter with a default value that was passed in from C. I compiled my code and moved it to a server that was a Dll from B and C. In my testing, this requirement worked fine. But when the part of the application that project A is involved in calling the test function was caused by an error (method not found). I couldn't figure out why A should break. I just recompiled the DLL from A and moved it to the server and the error went away.

My question is, why did I have to compile the DLL from A when I didn't change the way I call from A?