Could be any or all of the above.
Places where variations can occur:
Ruler used
Where ruler is placed on the fabric
How rotary cutter is held - perpendicular to the fabric or at a slant
Seam width
Thread used
Pressing technique
A handy test to see how you are doing:
Cut three strips of the fabrics you are using your (2 x 5 inches)
Sew them together with your 1/4 inch seam with the thread you plan to use
Press the way you usually press
Measure the unit:
It 'should' measure 5 x 5 and the center strip should have 1.5 inches showing
If it's wider than 5 x 5 - or the center strip is wider than 1.5 inches -
Your strips may be cut a bit wide
Your seam allowance may be too narrow
If it's narrower than 5 x 5 - or the center strip is narrower than 1.5 inches
Your strips may be cut a bit narrow
Your seam allowance may be too wide
Or it could be a combination of several factors - those are two of the easier things to 'fix'