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There are several things to keep in mind when looking at a character's experience.

First of all, remember that total experience is the total amount of experience that a character has gained and that next is the amount of experience required to level up.

Secondly, characters not being used in battle (chilling in the End of Time) gain 75% of the experience from battle. Additionally, if the experience gain for such a character is great enough to grant them two levels, the character will only receive one level and the next value will be that of the current level. There is no carry over of experience points. Thus, if Crono was out of the party at level 1 (not possible) and a battle was won earning 130EXP, instead of becoming level 4, he would go to level 2 and his next value would be 40. This is why your out of party characters are always at lower levels than the main ones you use the most.

[edit] Reading the Table

Note that the following tables contain the actual next value of every level (in the "Next" column) that appears when a character is at that level. Additionally, a cumulative next value ("Experience Needed" column) is listed denoting how much experience is actually needed to level.

For example, at level one you need 20 experience to go to level two. At level two, you need 40 experience to go to level three. At three however, you need 70 experience to level and thus the total experience needed to get to level four is 130. An easy way to read the table is, "To get to level L, I need a total of E experience; but to level up to L+1 I need N amount of experience" (|L||E||N).

Remember the point above about non-party character leveling and that Crono will primarily stick with this table for most of the game.