There could be 2 reasons:
Due to market price protection measures introduced by Exchanges, for some instruments, market orders are sent on the Exchange as limit orders.
In case of illiquid scrips, where the bid-ask spread is extremely high, there are chances that the market order gets executed at a freak price, i.e. at a price far away from the current market price. In order to prevent this kind of an adverse execution, Exchanges have defined a price range which is at a certain percentage above and below the last traded price.