Could it be your cutting which is slightly off? By cutting with the rotary cutter at 90* to the fabric you cut stripes slightly wider. Also the location of the 1.5 line on the ruler in relation to the fabric edge will also make a difference.
Position ruler exactly on the edge and use the rotary cutter at 45* angle to the fabric.
measure the stripes and squares and see if the begin the same size prior to sewing.
this was something I did wrong for ages and it made stripes a whisker. Larger each time.