There are many things that affect the taste/flavor and eating qualities of garlic. Variety and cultivar within a variety are the first thing but they are modulated by growing conditions and especially weather during the growing season and harvest. Moisture content seems to have a lot to do with it, too and recently excess precipitation caused garlic harvests all over the upper mid-west to have an unusually high moisture content and most varieties were mild for the year. Meanwhile, out west, more normal pungency was the general rule.
Conversely, during excess heat and drought during the growing season, the moisture content is low and the resulting taste is hotter or, more pungent. It's not anything the grower has any control over; it's nature. Growers can irrigate to relieve drought but what can one do to avoid flooding? Whatever the taste of the current crop in any given area is, it will be different in other areas with different growing conditions.
A bulb of garlic is a living thing; in fact, a family consisting of a single mother and her daughter cloves. The taste of garlic is always changing and almost never stays still. It is always in the process of changing from one developmental state into another as each clove is continually in the process of cloning itself into however many daughter cloves its DNA and growing conditions dictate, just like its mother clove before it.
Just by looking at it, you can't see any movement but deep inside, each planted clove's roots form and all through the fall and winter grow as much root mass as possible and with spring put up leaves and begin to form tiny cloves at the base of the leaves and over the spring, they begin to size up and the teardrop shape of a green garlic they start out looking like become the familiar bulb shape and the heat of oncoming summer forces them to mature.
The fact that garlic is in a constant cycle of change and growth explains a lot of its changing taste. Another great part is due to the moisture level inside the garlic. When a garlic is rudely pulled up from its comfy home a few inches below the soil surface, it has the most moisture it will ever have and thus the least pungent (hot) taste it will ever have. Many will be very mild at this state. As it dries down over a few months after harvest, it continues to lose moisture and as it does, the taste increases in pungency,or hotness when tasted raw though the hotness becomes f11lavor when cooked. How garlicky the cooked food will be is a function of how much the garlic is diced or chopped or crushed - please see the Cooking Garlic section for details but in general the earlier the garlic is added to the dish, the creamier and milder the taste will be and to maximize the garlicky taste of the finished dish, add the garlic just before turning off the fire and removing the cooking vessel from the stove to serve the food.