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What does 下一桁２ mean? In context, it is referring to some quantity and seems to mean under half, but not too low.
In this usage, why is ichi in kanji while 2 is a numeral? Can I replace the numbers with other numbers and get a different meaning? In the real world, when would someone want to use this phrasing instead of "x to y"?
So 下一桁２ probably means "the last (one) digit is 2" (e.g., 102, 65432). が/は seems to be omitted for some reason (I don't know why because you provided virtually no context).
why is ichi in kanji while 2 is a numeral
When you talk about a position of a digit, a careful writer does this to avoid confusion. For example, this Wikipedia article seems to be carefully written according to the convention.
Similar examples:
I am not sure what you are asking though, you may use the phrase 下一桁, 下1ケタ, 下1桁 for describing vehicle class number or the winning number of lottery.
This 宝くじ当選番号案内 says 下1ケタ 6番 (The image actually shows 2)
Probably the number of last digit so called 「チェックデジット」has been added to the number of credit card, identification number of drivers' license, etc. to avoid input mistakes.
According to コトバンク: 「チェックデジット」,
チェックデジットは一定の計算式により、各桁の値を用いて算出される。
例えば、「ISBN9784095012131」のチェックデジット（末尾の1）は、
偶数桁の値の和を3倍して得た75に、奇数桁の値の和24を加えた値99の下一桁を10から引いた値として得られる。
ただし、下一桁が0となった場合、チェックデジットは0となる。
I hope this is close to what you are wondering about.
