Why is a Grignard reagent prepared in excess relative to an aldehyde?
A Grignard reagent is prepared in excess relative to an aldehyde, because you seldom get a 100 % yield of the reagent.
The Grignard reagent is so reactive that you often get side reactions:
(a) Reaction with water
If your apparatus is not perfectly dry, you will get a reaction with water
(b) Reaction with oxygen
The Grignard reagent reacts with oxygen to form peroxides.
Reactive halides can give coupled products, similar to a Wurtz reaction.
All of these reactions reduce the yield of Grignard reagent, so you usually plan to prepare an excess to compensate.