For those using Tiger Brokers, I set a limit order for a stock at $30, but why after the order has been filled the average price become $38.03?

I believe $38.03 is the average price after commissions are factored in. So if you bought say 63 shares at $38, a $1.99 commission would be split over 63 shares. Meaning your average price would be (38*63 + 1.99) / 63 which would give you 38.03.

Hope this helps!