If you count 'em up. And yet, the pasuk states:
Rashi, Ramban with a bit of relaxing, that it was in Egypt) is that Yocheved was born between the walls. This is difficult, because she still was not listed; and the timing would need to be quite precise; and she would need to be 130 when she had Moshe. (Besides that I argue elsewhere that Yocheved was not actually a daughter of Levi, but rather a daughter from the tribe of Levi -- yes, there are difficulties, which I address.)
A second, better answer (see e.g. Rashbam, Ibn Ezra) is that Yaakov is counted, for this is the sum of the house of Yaakov. But it is still slightly awkward.
But I saw a great suggestion in the commentary of Aharon ben Yosef the Karaite, this week. Namely, that the Biblical style is occasionally to prefer whole, round numbers. He tells us to look in Shofetim perek 9, where there is also a case of 69 who are counted as 70, and where missing one person from the number does not matter.
Looking in the previous perek for setup, we have:
But we just said that there were a total of 70! The answer is that he actually slew only 69.
And it is not just Aharon ben Yosef the Karaite who says this. We have the much earlier Radak who says the same thing, and draws exactly the same connection between these verses. Pictured to the right is a clip from a beautiful Tanach with Neviim Rishonim, which features the commentaries of Radak and Ralbag on the side. It takes a while to get used to the ornate lettering, but Radak basically says ושבעים חסר אחד הרג. And even though it says 70, this is the minhag halashon in certain places to not to reduce the sum total because of one, such as -- and here is the reference to Vayigash -- that all the souls who came down to Egypt were seventy. Another example he gives is the forty lashes, for which we have a tradition that it is really a maximum of 40 - 1.
So this is actually a third classic answer, found in the work of a classic Jewish commentator, though we need to look non-locally.
Update: Another point to consider is that perhaps 70 is a "magic number" of sorts, like 40. Perhaps the reason taking one or two out from 70 leaves us with 70 there is that there were never really 70, but just a whole bunch of sons.