Row Index being Reordered After Merge.

Hi Community

 Just playing around with PowerBI, but I ran into something that I confused about.

Basically I am trying to shape the data so that for a given row i the previous two rows ( and their respective row values, I've only show year for example) will appear in the row as two new columns kind of like the lag window function used in SQL. I decided to use a merge method where I merge the table with itself using the indexing as the join paremeter. My Row Indexes are t (current row), t-1 (1 previous row) and t-2 (previous two rows). Represented below: 

I've only provided a small snippet basically there is a row index for every group

Group  Row t   Row t-1 Row t-2

A               0            -1         -2        

B               1             0          -1

C               2             1           0

This works for some group but noticed it doesn't work for some others after I expand. 
In one of the scenarios after merging I look at the output before expanding it shows what should happen, in this case for year 2015 the picture displays the possible new columns appended after merging.

But when I expand  - the indexes change and the ordering becomes messed if I want to append row t-2.

I think i would just have to reset the indexes when i do another merge (would like to avoid that) but I just want to know why would this occur.

Thanks!