There is no way (other than an error) that you were given 90 days on entry from the date the visa was issued. You would get 90 days from the day you arrive (For a Non O visa that is).
If you have a stamp that is 90 days from your visa issuing date then you need to get that stamp corrected.
A visa is valid from the day it is issued. Depending on what visa you have determines for how long it’s valid for and how many days you would get on entry.
A SETV is valid for 90 days. That means you have 90 days to get to Thailand and use the visa.
Upon arrival (as long as it’s before the expiry date) you will get stamped in for the number of days that visa allows (60 for the tourist visa). That’s called an entry stamp. It’s not a visa. The visa is what you got from the consulate or embassy you applied to.
Using a SETV it’s irrelevant if you enter on day one the visa is valid from or day 90. You’ll get a 60 day stamp.
With a METV that’s valid for 6 months from the day of issue. In theory you could enter in day 180 and get 60 days but the whole idea of a METV over a SETV is the multiple entry part.
Both visas “clocks” start ticking from the moment of issue. One is valid for 90 days to use. The other is valid for 180 days to use.
Two main factors in choosing one over the other. 1. Where your money is (or where you want to keep it) , 2. Medical Insurance.
With a Non OA you use local funds (not in a Thai bank - although they could be). Some don’t want to move 800k to a Thai bank where they will gain little to no interest. They keep the money and can use to invest in higher earnings. The visa is valid for one year, but with the right timing can be used for nearly two years. Each entry gives you 365 days, as long as your insurance covers that length.
The downside is the need for insurance. For those approaching or over 70 that can be very expensive or even unobtainable depending on medical history.
With a Non O visa you get 90 days on entry, no insurance required, but within 30 days you need to have opened a Thai bank account and move 800k into it. 30 days prior to your stamp expiring (some offices allow 45 days) you apply for the year extension and re-entry permit if needed.
Once you have a visa it’s irrelevant what flight day you put on the application. You can travel on any date within the validity of the visa. Obviously with a Non OA the clock starts ticking as soon as it’s issued so to get the nearly 2 years out of it you want to travel just after it’s issued. With a Non O visa it’s valid for 90 days so even if you enter 89 days after it’s issued you’ll be given a stamp for 90 days.