In the photos below you can see that a same design (with small adaptions) can have different signatures.
Probably the designer used the manufacturer for his production (objects signed with the signature of the designer), the manufacturer also used the designs for himself (signed with his own signature).
Another possibility is that A-quality pieces were signed with the signature of the designer and B-quality pieces were signed with the signature of the manufacturer. As it would have been a waiste of material and time to destroy these pieces.
Designers which decided to start their own workshop and leave the famous workshop they were working for took some of the designs they made with them and re-used them.
Another possibility are adapted signatures. Because the furniture made by artists like Gallé were / are making more money then "unknown" artist, signatures were "adapted".
If you have other examples please do not hesitate to post them!