The reason lies with the way in which the operation "plus" is defined and the way in which numbers are represented.
-123454
203
468
468
234 + 678 = 912
234 + 231 = 465
234 + 12 = 246
234 + 3747943 = 3748177
234 + 455 = 689
234 + 646 = 880
89+234=923
234 + 1000 = 1234