I may be all wet here, but isn't that the reason we square up blocks as we go. It looks to me that if you lay your ruler along one side, being very careful to leave a l/4 from the edge of your material and the points then trim, it would take care of the extra length in the middle square. That is the only place that looks like it needs trimming. Material is pliable, therefore, it could be that your machine handled it a little differently, the material is a different weave, any number of things. I don't think any two materials are alike, similar maybe, but not the same. If you have done everything everyone else suggested then you have a good block. I think you are doing great.
I understand as a beginner quilter you want to learn and "get things right". But remember, this is supposed to be fun so don't be too critical of yourself. Enjoy.