This issue is just a matter of inconvenience and in no way takes anything away from the lessons. The following are a few examples
 
Lesson 3.1 Asking for directions - the “next lesson” button links to lesson 3.5 Describing Things instead of 3.2 Describing People
Lesson 3.1 Asking for directions - the “previous lesson” button links to lesson 3.4 How’s the weather instead of Let’s go Shopping for Clothes.
Lesson 3.2 Describing People - the “previous lesson” button links to Let’s Go Shopping for Clothes instead of lesson 3.1 Asking for directions.
Lesson 3.4 How’s the weather - the “next lesson” button links to 3.1 Asking for Directions.
Let’s Go Shopping for Clothes - “next lesson” button links to Lesson 3.2 Describing People instead of Lesson 3.1 “Asking for directions”
 
I could go on but perhaps testing of the links is in order.