BSD-awk print() Behavior

Trying to wrap my head around what BSD awk is doing here. Although the behavior
is unwanted for this exercise, it seems like a possibly useful feature or hack
for future projects. Either way I'd like to understand what's going on.

I extracted a list of URLs from my browser's history sql file, and when
iterating over the list with awk got some strange results.

file_1 has the sql-extracted URLs, and file_2 is a copy-paste of that file's
contents using vim's yank-and-paste.

$ cat file_{1,2}
https://github.com/
https://github.com/
https://github.com/
https://github.com/

$ diff file_{1,2}  
1,2c1,2
< https://github.com/
< https://github.com/
---
> https://github.com/
> https://github.com/

$ awk '{ print $0 " abc " }' file_{1,2}  
 abc ://github.com/
 abc ://github.com/
https://github.com/ abc 
https://github.com/ abc 

The sql-extracted URLs cause awk's print() to replace the front of the string
with text following $0. file_2 does not. I used vim's `:set list' option to
view hidden chars, but there's no apparent difference between the two --
although `diff' clearly thinks so. Both files show this when `list' is set:

https://github.com/$
https://github.com/$


Here's more background if needed:

I extracted the URLs using sqlite3 like so:
for f in History-16768665*
do
        sqlite3 --bail $f <<-HEREDOC
                .mode csv
                .output ${f}.csv
                select * from urls where url like '%github%';
HEREDOC
done

Then tried to create a list of unique URLs using `sort -u' but it broke because
of special chars in the extracted lines (so it claimed). I used awk to get a
unique list instead:

for f in *.csv; do [[ -s $f ]] && list="${list} $f"; done; echo $list
awk '{ u[$0] } END { for (e in u) print e > "file_1" }' $list

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