Confused on two brands of tests

So 11dpo I got a BFN on a FRER. 12 dpo I got a very very faint line on an internet cheapie and a faint but clearly visible line on a FRER with the same urine sample. 13 dpo ever so slightly darker on the cheapie strip and about the same as the day before on the FRER. This morning (15dpo), the FRER is barely visible and the cheap strip is the darkest yet. What could cause the FRER line to get so much less dark while the line on the strip is still progressing? I am terrified because I had a chemical in December that started with a fainter test. Pics below show both brands with the same urine samples.