Retrieving a full record in CRM

When you try retrieve a full record from CRM using all columns = true would you be getting all the columns?

We recently checked this for one of our projects and strangely we were not getting all the custom fields with the result set. we were only getting a number of columns,

Then we tried another record from the same entity. The number of columns that retrieved was more that the previous occasion. However the columns that we received had a value assigned to each of them. 

So, it seems CRM when retrieving data only sends of the columns that have values assigned to them.