You will need to recall mathematics lessons for this one.
Lets's say you are a tri - mu/cl/wa - and you are level 1.
Now, you need: 2500 exp as mage, 2500 exp as cleric and 2500 exp as warrior to get to level 2.
2500 + 2500 + 2500 = 7500 exp total.
If you kill one and same mob and receive 250 exp for it, you will need 7500 / 250 = 30 such mobs (30 * 250 = 7500).
And in the end you wanna see 7500 exp in your score? 7500 exp is not comparable to level limits, because you need to know current level limits and somehow divide your total score among them.
Now we use a bit different method: we divide all your received exp by the number of classes you have and we show (as the consequence of this) only 1/3 of your total scored exp. So, in previous example your every kill of 250 exp will add just 83 exp to your total score and after 30 kills you will have: 250 / 3 * 30 = 2490. Almost the same (there is an error due to integer aithmetics). But your total score can be easily compared to what 'levels' command show you. So, when you finally score 2500 you'll receive all three levels: one for mage, one for cleric and one for warrior classes.
Of course, there is another aspect: not all classes have the same exp level limits. But we fill exp holes uniformly, and you can easily compare your total exp score to level limits directly. So, if you really want to know how much exp you have scored, you just multiply exp shown in score by the number of classes you have (x3) and feel proud of yourself.