Have you pressed them before you measured them to see if they matched?

Iron them! They should be the 8 1/2. But iron them it looks like they have not been ironed flat.

pineneedles4 you beat me to the answer.

Have fun with it when you get the measurements right
It's one of my favorite patterns

After you sew your 2.5 strips together, iron it and measure the height and that is the measurement you should use to cut the strip set into. It should be 8.5 but this could vary with the size of your seams. Just make sure it is the same measurement as the height of your strip set.

Ditto .... sew the WOF strips together with 1/4" seams, then measure that it is 8-1/2" .... if your seams are a thread or two off or not a perfect 1/4", then your strip sets will be different. Cut your lengths accordingly. Then be sure to keep the same seam width as you join your rail strips together.

I really like your colour combos ... will make for a nice homey- autumn-warm quilt!

sometimes you will find very small differences in the way you are pressing fabric. I always set the seam before I press it toward the dark fabric, which is usually the side I press to.
Good Luck!

Thanks. I actually had some left over fabric from some projects I made for my girls for this christmas and cut them into 2 1/2 inch strips. Always wanted to make a fence rail quilt so heres my chance.

Did you measure how wide the strip (block) is after sewing the four strips together(after pressing)?

Theoretically, it should be 8-1/2 inches if you started with 2-1/2 inch wide strips.

Sometimes the so-called "simple/easy" patterns - such as the log cabin block - can be a real bother to get to turn out "as expected"

Is it because one has the extra 1/4 inch edge on each side? the other one 8 1/2 inchs?