Back in the Philippines, we were taught that our official languages are Filipino and English. But some people tend to say "Tagalog" and English. A lot of foreigners and locals (Filipinos) tend to do that way. You would say that it's a common "mistake" . But here's the thing: Filipino is actually a Tagalog-based language as given to us as a national language in order to communicate with other fellow Filipino citizens. And here's how it goes...
The Philippines has about 182 languages: 19 are well recognizable and 4 are extinct (no exaggeration right there, I Googled it). The constitution decided to have Tagalog as its foundation but it is later changed or developed by adding few words from other dialects and became Pilipino.
Pilipino is the strictest form of the language here and there are many technical reasons why but primarily if one were to use Pilipino you are not supposed to use borrowed words from foreign languages like c, x, f, and q, it only speaks in the purest Pilipino words you can find which none of us can't do that anymore (they are sarcastically call it "deep" words) .Later, few more letters were added after the Spanish cultural influences and the American occupation.
To make it clear as an example: the letter/sound of "F" was originally non-existent, that's why Filipino (non-strict form) is spelled with an F. It has also been more politically appropriate that the Philippine national language is referred to as Filipino and not Tagalog.
"So why make it a big deal?"
For starters, I'm not a native Tagalog speaker, I am Visayan yet I have no hatred or sense of despise towards my fellow countrymen......a little......BUT! It is extremely important to learn this distinctions because doing otherwise would be a disservice to the rest of the other languages, along with Tagalog, that are the native tongues of the vast majority (almost 90%) of Pilipinos such as Cebuano or Bisaya, Ilokano, Hiligaynon or Ilonggo, Waray, Chavacano, Kapampangan, Bicolano, Pangasinan an etc.
This is also to avoid any offence to non native Tagalog speakers because it may be an insult or create an unfairness towards non native Tagalog speakers. (we're a little bit sensitive on it)
I mean- for goodness' sake we're a group of islands and the diversity and regionalism is ridiculously high, we all came from different tribes, beliefs and cultures but the bloody constitution is the only one that keeps us as one nation. If you say that our language is Tagalog and not Filipino, then you are referring only the Tagalog region and not the whole country itself.
In other words, be cautions when you are talking to a Filipino. Not all of us are from the same region :)
References:
https://www.smartling.com/blog/tagalog-filipino/
http://www.bakitwhy.com/articles/difference-between-languages-tagalog-and-filipino
https://www.quora.com/What-is-the-right-way-to-refer-to-the-national-language-of-the-Philippines-Filipino-or-Tagalog-Is-there-any-difference-in-using-these-two-names
