Why was Enfield surprised to see the name of the writer of the cheque?

"The mysterious man gave Enfield ten pounds in gold and a check signed by a very respectable third party, Dr. Henry Jekyll. Fearing the check was a forgery, the doctor, Enfield, and the family forced the man to stay in their company until the banks opened and the check could be cashed. When the banks opened, Enfield cashed the check, and was surprised to find it valid. Enfield could only imagine that the mysterious man had possession of the check as a result of blackmail. Throughout Enfield's narrative, he does not name he mysterious man. Finally, Utterson asks the man's name and Enfield reveals it was a Mr. Edward Hyde."