We may want to report errors that occur not just on the line numbers of modified lines, but also errors that occur one or two lines after a deleted or added line. This is because whether a line is an error can depend on lines previous to it. The worst that can happen is that some errors may be reported that are not the fault of the contributor (which is not necessarily a bad thing). See here for a few basic examples: https://bugs.webkit.org/show_bug.cgi?id=37122#c14