Tos shita on mehadrin min ha'mehadrin - a question that has me stumped

The gemara writes that there are three possible levels to fulfill the mitzvah of neiros Chanuka:

1) ner ish u'beiso - one candle lit every night for the entire household

2) mehadrin - every person in the household lights one candle each night 

3) mehadrin min ha'mehadrin - add one candle every night.  Tos' and the Rambam disagree whether this level builds on the level of mehadrin, i.e. every person in the house lights and adds an additional candle every night, or whether this level builds on ner ish u'beiso, i.e. one person in the household lights and adds an additional candle each night

Tosfos argues that the latter interpretation must be correct.  The whole point of adding the additional candle each night is to demonstrate which night of Chanuka it is.  If every person in the household lights, there is no way the viewer can figure that out.  If, for example, you see 4 candles burning in the window, does that mean it's the first night of Chanuka and there are 4 people in the house each lighting, or does that mean it is the second night of Chanuka and two people are lighting, or is it the fouth night of Chanuka and there is only one person lighting?  The only way the lighting can reflect the day is if we assume one and only one person in the household lights.

I'm not interested in how to answer this question and justify our minhag.  I'm interested in how minei u'bei Tosfos makes sense.  Even if mehadrin min ha'mehadrin means only one person per household lights, still, how can I know what night of Chanuka it is?  Remember, there are three possible levels of fulfilling the mitzvah.  If I see 4 candles burning in the window, how do I know that that home lit mehadrin min ha'mehadrin and it therefore is day 4 of Chanuka?  Maybe in that household they only keep the mehadrin level.  Mehadrin means every member of the house lights one candle.  4 candles might means four people at home, each lighting one candle like the mehadrin custom?  Tos' hasn't solved their own problem!

I've learned this sugya multiple times over the years and for whatever reason, this never occurred to me before.  It's a simple pshat question and I am stumped.  I keep thinking I must be missing something, but I've asked a few people and so far no one has come up with a real answer.  To say that Tos' assumed everyone lit mehadrin min ha'mehadrin seems like a big dochak.  The gemara gives three levels and Tos' comment has to be taken in that context. Anyone have a good idea?