The magnetic field lines are not horizontal in most places in the world. They are more or less horizontal around the magnetic equator, and approach vertical as you approach the magnetic poles.
IIRC, Suunto compensates for this and has different versions of compasses for the different areas of the world (actually for different approximate sign and magnitude of "dip").
Perhaps you have ones that are optimized for a different area. For example, a compass optimized for UK and Scandanavia would have compensation for a significant amount of dip. With the much less dip in Singapore, you would see the compasses tilted a bit.
It doesn't really affect the azimuth/direction accuracy. You just need to tilt the compass a bit.
p.s. As Kraken suggests do move them away from each other before coming to any conclusions since they will interfere with each other.
