The walls of a top bar hive are sloped inward towards the bottom so the bees will build less

comb attachment to the walls of the hive. This is the reason for the angle of the sides of the hive. If the hive were a square box the bees might attach the comb they draw out along the entire side of the hive. This would make it very difficult to harvest the honeycomb from the hive. 

Bees attach a few inches of honeycomb to the sides of the hive,
this is called, brace comb.

The honeycomb is quite heavy when the whole
comb is filled with honey. This is one reason the bees attach
this heavy comb to the sides of the hive.

 A closeup of the honeycomb, the darker area in the cells
is honey!

You can see attached comb on the left of the picture and

unattached comb which is brood comb on the right. Brood comb

(the comb were the queen lays the larvae) is lighter and the bees

usually do not attach this to the sides.

On this beautiful pulled honeycomb you can see the slight

angle of the combs shape. This is due to the angle of the

hive and how the bees draw out their comb.

These are bees making a "chain", they hang on

each others legs and arms and form a haning chain to create

the shape of the honeycomb. 

Think of how a chain necklace hangs, gravity pulls the
chain into a specific shape

Closeup of bees haning in formation

 A bent coat hanger,made into a "scraper", scrapes the

sides of the hive where the honeycomb is attached

The honeycomb is gently removed from the hive!