First off, I'd like to point out that your Scripture from Leviticus 27 is a different kind of tithe than a standard one. Leviticus is talking about special vows and gifts. Leviticus 27 is discussing vows up to the last part, where it then goes into the topic of tithes. What you are referring to as a "non standard" tithe was one of three mandatory(for those in the agricultural and livestock occupations) tithes, known as the Levitical tithe. Notice the text doesn't suggest it was optional tithe for those who were in the livestock/agricultural professions.
Well, I disagree with your interpretation of what Leviticus 27 saying. The entire chapter is talking about the special vows and offerings, as is indicated by the closing verses talking about redeeming the specific tither. So again, I disagree with you.
We give 10% of my gross paycheck right off the bat. But in addition to that, there are various other offerings, especially from the agriculture on our property (figs, eggs, veggies, etc.) So back to my original question, then, of why one type of tithe but not the others?
I don't know what you're asking here.
And the "best" of money isn't always a quality thing. For those with an agricultural tithe, it does mean as perfect an offering (animal, grain, etc.) as possible. Offerings and tithes are two different subjects. What Biblical teaching on tithing mandates the "perfect" of your crop/livestock?
But when it comes to money, it has nothing to do with quality, rather the first. Based on what Biblical teaching?
The perfection standard for offerings comes from the criteria in Leviticus 1-6. In regards to tithes, I'm not aware of a specific Scripture that says the standard tithe has to be a perfect animal, or maybe even a first. Perhaps there is one that I'm not aware of right off the bat. So I guess that it is my own personal standard.