Because we are still taught the Roman numeral system under rules introduced during the Middle Ages which converts 99 into Roman numerals as XCIX (90+9 = 99) but these rules were made in a haphazard manner centuries after the fall of the Roman Empire.
In all probability the Romans themselves would have calculated 99 as LXXXXVIIII and by placing I to both sides of these numerals simplified them to IC (-1+100 = 99)
In fact the Latin word for 99 is "undecentum" which literally means one from a hundred.
In todays modern notation of Roman numerals they represent 99
99
99
The number 49 in Roman numerals is XLIX and the number 99 in Roman numerals is XCIX
The Roman numerals X-XCIX represent 10-99
In todays modern notation of Roman numerals they represent 99
99
99
The number 49 in Roman numerals is XLIX and the number 99 in Roman numerals is XCIX
The Roman numerals X-XCIX represent 10-99
XCIX is the modern standard for 99.
99+1056 = 1155 or MCLV in Roman numerals Today we write out 99 in Roman numerals as XCIX but the Romans themselves would have probably wrote it out simply as IC (-1+100 = 99)
XCIX - XIX = LXXX
The Roman numeral for 99 is XCIX
Nowadays we would convert 99 into Roman numerals as XCIX But the ancient Romans probably chose the simpler version of IC
C=100 X=10 I=1 So it is 10 before 100 (=90) plus 1 before 10 (=9) . Thus the answer in ARABIC numerals is "99", while of cause XCIX in ROMAN numerals is "XCIX"
In today' notation of Roman numerals it stands for 99 but during the Roman era the equivalent of 99 in Roman numerals would have probably been written out simply as IC (100-1 = 99)