Password Strangeness

I'm in this group. This group has a web tool. This web tool is made with ease of use and trust as cornerstones, so there's one password. That password, for purposes of discussion, is Abc123. Be it noted that I remember the letters and numbers, but not necessarily the cases.

While waiting for the bus to work, another member of the group wanted access to the web tool, and emailed me for the password. I did some checking on my Android phone and told him that the password was ABC123, because that worked with my phone.

When I got to a computer, we did some testing and found that neither abc123 and ABC123 before I checked the password configuration and found the right one.

So, why, when I put the password into this tool, did it take ABC123 as Abc123.