In Georgia at least you have to manually scratch off a series of code numbers at the very bottom of the ticket and key it in to verify if it's a good ticket or not. Just scanning it means nothing; the UPC codes are all the same. There is no risk of fraud, unless the lotto company is intentionally putting out bad tickets.
Also, were your tickets $1-$3 tickets or $5 and up? Because the $5 and up tickets give you a much better chance than the former, which have higher odds against the player. Which is natural because cheaper tickets are sold much more frequently and to have them win at the same rate as the more expensive tickets would bankrupt the company.
True story, I've been trying to get rid of the same $20 since November....
-Turn in ticket
-Get 4 from the winning ticket
-scratch/lose
-scratch/lose
-scratch/lose
-scratch/$20
-and time flows like a river, and history repeats itself.
It's ridiculous.
EDIT: About your suspiscions on the cashier; their lotto terminals may be able to display which tickets have won the most in recent times, so they can use that to factor the odds in their favor and buy tickets that seem to have a higher chance of winning at the time.