For a start, the words before the ones you mention are:
שֶׁבַע שַׁבָּתוֹת, תְּמִימֹת תִּהְיֶינָה
Seven complete weeks are only 49 days.
Secondly, we see other times in Chumash that a number means "until, but not including" that number. For example in Devarim (25:3):
אַרְבָּעִים יַכֶּנּוּ, לֹא יֹסִיף
We only strike him 39 lashes; all the up to - but not including - 40.
That's also the simple explanation for the 70 souls that came down to Egypt with Yaakov: Breishith (46:27)
כָּל-הַנֶּפֶשׁ לְבֵית-יַעֲקֹב הַבָּאָה מִצְרַיְמָה, שִׁבְעִים
If you count them you only get to 69; all the up to - but not including - 70. (And yes, there's a known Medrash that the 70th was Yocheved born as they came through the gates.)