My guess is, that it's exploiting the basic synaptic nature of the brain. The 12 numeral automatically kicks in the association with months, which in turn split

My guess is, that it's exploiting the basic synaptic nature of the brain. The 12 numeral automatically kicks in the association with months, which in turn splits the sum to 12 x $1, or one dollar a month which seems low enough to feel at ease with. Seems to be intuitive enough to happen subconsciously. The other price points just appear as one fixed sum in the mind, which is much less likely to be spent on a whim.