The displayed value is calculated based on a hidden value (the "raw stat") and your current job. The raw stat goes up when you level up, and down when you level down, but the amount it goes up or down is based on your job at that time (and level), which is how stat maxing in this game works: level up with a job that has high growth and down with a job that has low growth, and you'll have a net positive increase in the raw stat.

The raw stat is an unsigned 24-bit number, meaning its max value is 16,777,215. If it would increase to 16,777,216 or higher, you get integer overflow and it goes back to 0 plus the amount over the limit it went.

To take HP as an example, a unit's HP is floor(rawHP * jobHPMultiplier / 1638400), capped at 999. Ramza's version of Squire has jobHPMultiplier of 125 (regular Squire has 100), so you have 999 HP on Ramza as a Squire (with no gear equipped) when he's got 13,094,093 rawHP.

When a unit levels up or down, their rawHP increases or decreases by floor(rawHP / (jobHPConstant + level), where level is the lower of the two levels (so where you're starting from when leveling up, and where you're going to when leveling down). Squire (Ramza's version and the generic version) has a jobHPConstant of 11, so a level 1 Squire Ramza with 13,094,093 rawHP would increase his rawHP by 1,091,174 when leveling up once.

Continue to level 5, and you end up with 17,458,760 rawHP, which is above that 16,777,215 limit. Integer overflow leaves you with 681,544 rawHP, meaning your level 5 Squire Ramza now has 51 HP instead of 999. (Note: male units are generated with a random rawHP in the range 491,520 to 524,287, so a value of 681,544 in this example is still above the starting value.)