It's not unusual to see 100 feet of difference, on the low side. Very common here (tropics). It's not unsafe either because all it means is that your altimeters will show you at DA before you actually get there.
A different story if your altimeters are over-reading though because, by the time they read your DA, you'll actually have busted it by the amount of the over-read.
All else being equal, I like to monitor the Radalt because it catches both situations.
As for the actual reason for the situation... probably something to do with the check altitude being based on a standard ISA day and your conditions not being the same as that. Then maybe factor in a bit of error in the QNH that you've been given. Also maybe - and I mean maybe - factor in a bit of PEC.