Pain in the Gas(oline)

The cost of 87-grade gas (a.ka petrol) at a QuikTrip in Tempe AZ yesterday: 2.099 a gallon
Come to think of it: its a weird pricing thingie..what is the origin of this? What does that extra digit signify? Why not just round it up to the next higher even value?

>>Rounding up?:"Bata pricing"*** a.ka psychological cajoling of a harried/hurried customer.
>>Tax?: Maybe, though I didn't find one reliable source to ratify that fact. Alternatively..a message board discussion on Snopes.com the urban legend reference point.
>>Interesting note on gasoline volume expansion in summer. Relevant to places prone to temps over 90 F in the coming months.

***Indian slang. Same theory of pricing a $20 product at $19.99 or $19.95