2 answers, based on asking people the same question:
1: Many Ashkenazi shuls do not say ברכו at the conclusion of the Weekday Shacharis prayers on Mondays and Thursdays or Rosh Chodesh.
They (apparently the Talmidei haGro) borrowed the ברכו at the end of Shabbos Shacharis from the Sefardim, in case somebody came late.
But on days when there is Krias haTorah, the latecomers will already have heard ברכו, so there's no need to add it.
2: Many Ashkenazi shuls have adopted various Minhagim in order to accommodate people with various Mihagim. (I.e Sefardim, Teimanim and Chasidim.)
Included are ברכו at the end and saying the י"ג מידות before Tachanun, at least on Mondays and Thursdays.
But on Shabbat they keep to the original Nusach Ashkenaz.