Mein Bruder is schlechter als deiner.

In lesson 6.5 we have:
Mein Bruder ist schlechter als deiner.

Since the two items being compared take the same case, why isn't it:
Mein Bruder ist schlechter als dein.

In nominative case you'd say "dein Bruder," not "deiner Bruder," wouldn't you?

I think what is going on is that we have a possessive pronoun here and possessive pronouns decline like dieser, so deiner is nominative masculine singular and agrees.
Thus- Mein Bruder ist schlechter als deiner

Is that right Julia?

Hallo RobertD50 und sfpugh,

Thank you very much for your question, RobertD50, and you are right, sfpugh, we have possessive pronouns here!

You can split possessive pronouns into two categories: Dependent possessive pronouns and independent possessive pronouns. They also take on different forms depending which category they fall into as well as the gender and case they have to agree with.

Dependent possessive pronouns
These are placed in front of the noun, hence they are sometimes called possessive articles, for example:

"Mein Bruder ist schlechter als dein Bruder." - "My brother is worse than your brother."
"Mein Auto ist schneller als dein Auto." - "My car is faster than your car."
"Meine Tochter ist größer als deine Tochter." - "My daughter is taller than your daughter."
"Meine Eltern sind älter als deine Eltern." - "My parents are older than your parents."

Independent possessive pronouns
These are a bit different. They are used as a replacement for the noun which was previously stated. Since they are an independent element of the sentence (not used with a noun) they decline differently, for example:

"Mein Bruder ist schlechter als deiner." - "My brother is worse than yours."
"Mein Auto ist schneller als deins." - "My car is faster than yours."
"Meine Tochter ist größer als deine." - "My daughter is taller than yours."
"Meine Eltern sind älter als deine." - "My parents are older than yours."

I hope this was helpful.

Viele Grüße
Julia

Thanks for the explanation.

Thanks for the explanation Julia, there is an interesting confusion in terminology here.
I have seen what you refer to as dependent possessive pronouns referred to as both possessive adjectives and possessive determiners!

What to Germans call them in German?

Hallo sfpugh,

you are right, this would be confusing! 
Dependent possessive pronouns can also be called possessive determiners and possessive adjectives because they are placed in front of the noun.  

Germans usually just call them "Possessivpronomen" - "possessive pronouns"  which can either be used as a determiner (dependent on a noun) or as a replacement for the noun.

Grüße
Julia
 

Thank you Julia. So abhängig Posessivepronomen and unabhängig Possessivepronemen are not formal grammar terms in German.

I think  possessive adjective versus possessive pronoun is probably the least confusing for English speakers. :-)
 

I will keep that in mind!

Tschüss
Julia