Quite right the the elr is less than the dalr, but elr is also varible depending on the actual conditions. Using the dalr will give you the "worst case" scenario, i.e. the lowest cloud base in theory. The actual cloud base will probably be higher as elr is less than dalr. However the aim is to pass the exam and then get experience in real life scenarios. The exam obviously uses dalr therefore the "correct" answer will only be found using dalr.