stop loss order..theory vs practical..very confusing

hi all i am a new bie here, however efforts i take to study stop loss orders i am not able to understand . for eg todag on hdfcsec i tried for sell stop loss

1) LTP 872.95
Price 871
trigget 874

this txn was rejected with message trig must be greater than equal to price..which price?

2) ltp 112.40
price 112
trigger 112.50

this got executed
so what was wrong in first case.

also in buy stop loss which i also tried error was trigg should be less than or equal to price

kindly advise

stop loss order..theory vs practical..very confusing