Armen Hovakimian has made a map were we can see the differences in the distribution of R1b and I2a2b in LBA-EIA period. Red is the R1b the blue is the I2a2b. As You can notice the I2a2b is more frequent in Sevan basin than elsewhere.
This structured distribution can be due to initial TVC groups random founder effects. But in most likelihood it is related to secondary developments like the emergence of Sevan-Artsakh culture (known as Sevan-Uzerlik ) Sevan-Artsakh is few hundred years younger than TVC and it is usually assumed that it descend from it. The current data supports that theory.
Later after 1500bc Lchashen culture emerged and most of this local developments disappeared, assimilated into more homogenous Lchashen sedentary culture.
The reason why modern Armenians have more R1b than I2a2b is due to this localized distribution of I2a2b. While R1b was widespread all over historic Armenia the I2a2b had a founder effect mostly in Sevan basin. Even today the highest level of I2a2b in Armenians and even in the world is found in Gardman Armenians who lived in eastern parts of Sevan. More than ~8.5%. As for modern inhabitants of Sevan basin. They descend in large part from Bayazet, so it's not surprising that they do not have the highest level