At this time when you try to insert a spreadsheet form into a page the associated spreadsheet is inserted instead. Google knows about the problem. Until it is fixed a workaround is to paste in the form URL.
Edit the form and click More actions and select Embed. At the bottom you will see:
You can view the published form here: https://docs.google.com/spreadsheet/viewform?formkey=dEZmZTlJbUtWLUhONjM3OS05VzFwUHc6MQ
Copy the URL. Then when you insert the form paste that URL in the box at the bottom labelled Or paste a web address here