Are you official?

No (or not yet)

Are you affiliated with any government or private organization?

No. STOMP is a personal project. :)

What are your sources and how is data gathered?

Data from DOH's COVID-19 Data Drop (also here) is used as base and augmented with information released by Local Government Units that are encoded and validated by volunteers. Counts for provinces, cities, and municipalities use official DOH data. Counts for barangays use social media posts by LGUs.
No personally identifiable information about the confirmed, suspected, or probable COVID-19 cases are collected or exposed to the public.

Why is it that when I total the sum of all cases in provinces, cities and municipalities, or barangays, the sum is not equal (or less than) the total for the country?

The cases with associated province or city/municipality information isn't complete. They also don't release barangay-level counts. This is why we are doing what we can to supplement this data and paint a better picture (or draw a better map) of COVID-19's geography.

Why are you doing this?

Because we need granular, good-quality, open data about COVID-19 in the country so that people can use the data to make evidence-based, data-driven, informed decisions.
COVID-19 is beyond any one individual and affects us collectively. It transcends color, race, political affiliation, geographical regions, administrative boundaries, etc. We need to do what we can to help each other overcome this challenge together.

How can I help?

Stay at home. Wash your hands. Help the less privileged. You can also volunteer to help update and validate the municipal and barangay-level COVID-19 data. :)