Should amount of citation/references be taken into account when grading an assignment? As I don't like looking up the internet and read all the materials or references and all that when doing assignment, thus I try to pay full attention during lectures and/or tutorials. Therefore, my assignments are often long and with minimal to no plagiarism detection rate as all the contents are written by myself from my own knowledge - not paraphrasing. Perhaps I'm not until the stage where I need to often look around for more materials I guess? I'm currently still a year 2 undergraduate.
There's sometime where I have to lookup on the internet so adding references is unavoidable in this case. But in general, I ended up having my assignment to contain only a small amount of references, maybe 3 to 5. My plagiarism detection result is usually just around 0% to 1%. Or even in most cases, I just refer to my lecture notes and that's it.
But recently as I enter year 2, this started to give me problems. I just ended my first semester and was given lower than usual marks for my assignment, with the lecturer commented that the amount of references and/or citation is insufficient, although the plagiarism checker finds no match. Hence, marks are being deducted.
Therefore, my question is:

Should the amount of references/citation be taken into account when grading?
Let's say, this idea or knowledge is on my mind, or maybe I learned it from lecture, and it is sufficient for me to complete my assignment, without the need to actually refer back or anything. Should I take the extra effort to find out the related sources and reference or cite them?
Or, is my way of doing assignments actually wrong and improper? Should finding external sources of information as references when doing assignment is actually a must?

I also need to clarify that all my assignments are getting reasonably good grades.
Please do correct me if I got any idea and/or concept wrong.
Plagiarism is not about whether your work passes some % threshold on software that detects plagiarism.
Plagiarism is the representation of words and knowledge/ideas as your own. When you learn from a source, you need to cite that source when you write about it.
Over time, some knowledge becomes so commonly known that it doesn't need to be cited any more in most cases; for example, Newton's laws. Lecture content might sometimes fall into this category and sometimes not, you can ask your instructor for guidance on their expectations. However, most of the content in a paper you write at a college level will come from a source you need to cite, unless it's work you've physically done or your own unsupported opinion.