Checked today's dollar rate, then got a different number at the bank? Neither is wrong; they are different kinds of rate.
Latest rate and history: USD to THB.
The Bank of Thailand publishes the weighted-average interbank rate and the average rates Thai commercial banks use with customers, every business day. Those averages have a buying and a selling side; the mid rate is halfway between the buying-transfer and selling rates. We use it because it favours neither buyer nor seller.
Banks and booths add their own spread, cash and transfers are priced differently, and markets move during the day. Use the reference rate to plan, then check your bank's rate before exchanging.