Health is a floating point, but almost always treated as an integer. For instance, a horse with 30 max health and 29.001 current health will have 15/15 hearts full, because there are few damage sources that could kill the horse without dealing 30 damage.

But, a similar horse with, say, 29.3 max health will only have 14 hearts in its health bar, despite also being able to survive 29 full points of damage. If it had a 15th heart, it would be full.

This may have been introduced in 1.19.4; I don't recall breeding horses prior to it and getting any horses with non-integer max health.

To replicate:

• summon a horse
• do /attribute @e[type=minecraft:horse,sort=nearest,limit=1] minecraft:generic.max_health base set 29.3
• note the health bar with 14 hearts
• if you like, deal a full heart of damage to the horse, and all 14 hearts will still be full (27.3 current health is rounded up and displayed as 28)