There are several possible reasons for the start and stop points to not line up exactly. These are due to how recombination of DNA across generations and the Family Finder matching process work.

The first reason is that part of the segment recombined slightly differently from one generation to the next. This would result in the matching segment being slightly smaller.

The second reason is that a child might pick up a small segment of coincidentally matching DNA from the other parent. This would result in the matching segment being slightly larger.

The third reason is that the microarray technology used is not 100% accurate for all SNPs (data points) tested. By chance, several matching SNPs at the start or stop of a block are wrong for one person in the match. In this case, the program could stop the matching segment one block sooner than is real. Alternatively, it could allow matching to continue one block further than is real. The Family Finder matching process takes several steps to prevent this testing technology drawback from affecting the quality of the overall matching process. However, it will cause small variances in individual matching segments when you compare many relatives.