Is the metric of new users versus recurring users reliable? How do I value user loyalty?

I have a quick question regarding new users versus recurring users.

I know that new and returning users generally do not join users because of duplications, and "users" could be considered as the deleted version. My question is: when I look at my data for each individual month, the proportion of new vs. returns is well above 30%: 80%. But when I look at my data from the creation of my website until now (which is 2 years), the proportion is 17%: 83%, clearly NOT the average of two years. Why is the set like this and if when I look at the data over time it's not the average, how can I know if the new users versus the returning ones are even reliable? How do I track user loyalty? Do you have other metrics that you recommend?

Thank you!!