If I understand it correctly, descent from the male line could go back several generations - meaning there might have been descent from a mere female (and a name change) at some point.
e.g. the male line entailment might go back to the grandfather of the current Mr. Bennet. The grandfather's younger brother would be the next male line. If he had only daughters, then the oldest daugher's oldest son would be next in line.
I hope this makes sense.
I suspect (if this is the reason) JA did it to show how unjust male entailments actually are (which would be typical of her sly satire). In this case a male who was from a female line would be given preference over the daughters from the direct male line.
e.g. the male line entailment might go back to the grandfather of the current Mr. Bennet. The grandfather's younger brother would be the next male line. If he had only daughters, then the oldest daugher's oldest son would be next in line.
I hope this makes sense.
I suspect (if this is the reason) JA did it to show how unjust male entailments actually are (which would be typical of her sly satire). In this case a male who was from a female line would be given preference over the daughters from the direct male line.