Merging table with a left outer join

Hi Everyone,

I have two table that I am trying to merge data in.

In my Travel Data table I have an Origin Code, which relates to the airport of departure.

In my Origin IATA Codes this is the Airport of departure code.

Merging these two tables with a left outer join, should give all the data from my travel data table with the updated information added from the Origin IATA codes Table.

In my travel data table every row has a Origin IATA Code (not unique) and all are valid Origin IATA Codes (unique), so it should be a straight match.

My data has 375109 rows and it says it matches 375109 rows.

When I update my changes

My total goes from 

To 

I don't understand why there is a difference.

I have compared the before and after using Excel and I can locate where a row has been duplicated as in the example below, but I can't understand why it has acted this way.

I am thinking that perhaps the join is wrong or the relationship?

TIA