It’s a way for the gas corporations to make more money. The link I gave up above send you to an article. Somewhere between the middle ad the end it shows you just how much that 9/10ths of a cent equates to for the company.
That’s weird, here in Australia we too have an extra fraction of a cent, but here it is just added in decimal form, so in the example, $3.85 9/10 would be expressed as 385.9 cents.