This Happen Becasue Bracket Order is a Limit order it will only execute at that price which you have mentioned.
So, when the Stock CMP reach at that price it get executed but If your buy/sell quantity is not available there it will buy/sell available quantity.
The remaining Quantity will be in queue waiting. When again CMP will go at that price which is your buy/sell price quantity get executed and this Create a multiple orders in order book (This all happens in fractions of seconds, because CMP not gonna stay at one price when there is volatility. )
What happen in Cover order?
Cover order is a market order in Kite (Limit order in Pi)
When you give a buy/sell for a qantiy it will buy/sell at recnt available price (it average the price)
Like if you buy at 100 it will buy at 100.05, 100, 99.95 and Average the Price.
But this not happen in Bracket order.