The issue occurs because of the default padding-left and padding-right of 4px, that is calculated within the 100% width. In order to remove it, I would suggest to create a CssClass for that case when you want the button to be 100%, and to remove the padding:
The padding is necessary for the button when width is auto to prevent text to be merged with the borders. When you set width in pixels for the button the padding is removed, but when it is in percents it is not removed. We have log that issue and it will be fixed. For the time being, you could use the solution above.
Regards,
Bozhidar
the Telerik team
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