Seems like a custom CSS styling issue. Have you tried changing the default theme to make the buttons align the same?
You can create a table in a row of your main table. Take a look at my popup form
If have merged all cells on my last row and added a table with 3 cells. For my form I use the widths: 16;25;59
In order to make sure the layout stays similar to the normal dataview layout you have to add a property to the style setting of your merged cells: padding: 0px;
Change the class of each of the three cells, the property class should be changed to: MxDataViewCommandsTD
Below is an example of what my form looks like in the modeler and in the client: It is possible to give those buttons the exact same layout as a normal save/cancel button, but you can only do that with some css classes in your theme package.