Generally the slogans on the front of a T-shirt are selected to appeal to a large number of people, because this is good for sales. Occasionally, you will see a T-shirt that stands out because the slogan is very specific, and would probably only appeal to a handful of people. These T-shirts can can be called oddly specific.
Most of these T-shirts are produced by companies that download data from Facebook (or other social media sites). The company uses an algorithm to use selected data from those social media sites to create an image of a T-shirt, which is then advertised to you, when you are using the internet. You may then see a T-shirt advertised on the internet that matches your name and/or occupation, and some personal information that you may have shared on one of those sites. Enough people are curious enough to buy such a T-shirt that the company makes a reasonable profit.
The sort of information they may collect about you is:
recently promoted/laid off/new job/etc
complaining about parents/children/spouse/relatives/etc
country/state/town of birth/residence
etc., etc., etc.
For example you may see a T-shirt such as:
I am Rick a 45 year-old Grandad from Alabama who loves the Red Sox and
Hates Ford Cars