This is definitely true. I generally put up small one off projects where I implement an idea I'm interested in, as opposed to long-term libraries or applications that are intended to be used by others. For the longest time I wasn't technically skilled enough to contribute to other people's code, but now I think it's more about a lack of time and interest for me. I'll definitely make a deliberate effort to collaborate and work on other people's projects going forward. I'm curious to know though, what do you mean when you say that I "show an understanding of some important concepts in [my] work"?