Should a group header in a grid header setup span the width of the component when the sub-columns are config'd with flex 1?

I'm seeing each sub column at 39 px in IE and Chrome.

Code:
Ext.widget('viewport', {
	layout: 'fit'
	, items: [{
		xtype: 'gridpanel'
		, title: 'Title'
		, columns: [{
			text: 'Header'
			, columns: [{
				text: 'Name'
				, dataIndex: 'name'
				, flex: 1
			}, {
				text: 'Name'
				, dataIndex: 'name'
				, flex: 1
			}]
		}]
		, store: {
			fields: ['name']
			, data: [{
				name: 'Bob'
			}, {
				name: 'Carl'
			}, {
				name: 'Bob'
			}]
		}
	}]
});