That is because the similar letters are grouped together on purpose. Here is the original alphabet which didn't have any dots:

أٮ ح د هـ ور ح ط ى ك ل م ں س ع ڡ ص ڡ رس ٮ ٮ ح د ص ط ع

Then they added the dots to make things easier.

ب ت ث you wanna know how to tell these three letters apart?

:wtf-am-i-reading:

The ‫(b sound) ب‬ has the dot below its shell and the word “below” starts with the sound b. It's transliterated as b

The ‫ (t sound) ت‬has two dots above its shell and the word “two” starts with the sound t. It's transliterated as t

The ث (th sound) has three dots above its shell and the word “three” starts with the sound th. It's transliterated as th