stain inconsistencies

I’ve recently purchased my first home and am having the hardwood floors sanded, stained a darker colour and sealed with solvent based poly. The contractor seems super competent and spent multiple days sanding. I then chose a stain and they went to work. Today I just popped in to have a look, and...

I had the opportunity to mill some beautiful old ash into 5 inch boards. Some are 16 feet long. The floor installed very well. We started testing some stains, and some of the boards are becoming much darker than the majority. Of course the darker the stain we tested the worse the problem...