This seems to be very commonly confused so I wanted to try and clear it up for those who don't understand.
Lets say we're talking about a mage, and you say to train DD/hp as 3:1
This obviously means we want DD to be three times as large as hitpoints- but how do you train this? Most will say: 1/3 hp, max dd. Wrong. Training 1/3 into hitpoints leaves 2/3 as the remainder. 2/3 is only 2x of 1/3. Thus training 1/3 hp 2/3 DD is a 2:1 ratio.
To correctly achieve 3:1 ratio in this case you would need to train 1/4 hp and 3/4 DD.
If this still seems confusing, consider 2:1. 2 to 1, DD must be twice as large as hitpoints. Immediately you must realize training 1/2 is incorrect as both stats would be even. To achieve 2:1 you must train 1/3 hp and 2/3 DD.
The only reason I bring this up is so that when you give advice to a newbie, its probably better to give ratio recommendations as fractions that are similar to the format we use in training. Otherwise they'll probably train it wrong as described above and your advice will be wasted.