I believe there are better ways to do this, but this works.
First download the Ubuntu manifest file for your Ubuntu release
wget -c "releases.ubuntu.com/$(lsb_release -r -s)/ubuntu-$(lsb_release -r -s)-desktop-$(dpkg --print-architecture).manifest" -O ubuntu.manifest
Then generate the list of packages you have in your system and save it in a file called installed
dpkg-query -W -f='${binary:Package}\t${Version}\n' > installed
Then copy and paste this python code to a file naming pkg-diff.py
(or whatever name you want)
f = open('ubuntu.manifest', 'r')
default = []
for line in f:
default.append(line.split('\t')[0])
f2 = open('installed', 'r')
installed = []
for line in f2:
installed.append(line.split('\t')[0])
extras = list(set(installed) - set(default))
print("\n".join(extras))
Finally execute the python script using the command in a terminal.
python3 ./pkg-diff.py
It should give you the list of packages you installed additionally.
Note: All files should be in same directory.