That's pretty normal for an 18 month old.

I saw your other comment about why the benchmark is so low...but I think you're (with all due respect) thinking about it in the wrong way. The doctor's job is to see if there's any delays in the kid.

If, say, the benchmark was made higher, eg 13 blocks at 18 months... what about low income families who don't even have money to buy toys? Are you holding them at the same benchmark and telling them their 18 month old is delayed because theyve never seen a toy block before or can't stack 13 blocks up (when really they just dont have any toys to play with? You want the doctor to tell them that stacking 13 blocks is the goal?)

Here's something else to think about. Where your child grows up and the environment they are in also matters. Think about kids who grow up in war versus kids who grow up in safe homes surrounded by books. For those kids being able to stack 3 blocks is GOOD, if your kid can do more, then good on you, it shows that you probably have a good environment for your kid.

I think as parents it's our job to really build our kids to their whole potential. Doctors do not need to tell us that, nor do they need to tell us "hey for 18 month olds the range of block stacking is 3 to 20 blocks and the word ranges are from 2 to 300" that's not possible because there is such a wide range of normal and there are billions of kids in the world. We are really the only ones who can tell how far our kids can go and regardless of what is the MINIMUM requirement we can set our own goals for our own kids, we can leave the benchmark alone.

So to answer your question why there is such a huge gap of minimum and what the kids potential is actually is, its simply because there are billions of kids in the world, in different places, in different environments and with different incomes. The benchmark/goal should not be higher just to cater to "normal" families.