Because you are viewing the website on a low resolution screen. The sidebar is always expanded beyond 1200px width.
You can’t fit in middle content + both sidebars on 1024px width with the way it’s designed.
i have modify sidebar content padding width custom option for 10px padding.
file: theme-options.php
row 921
before:
array(
'value' => '30',
'label' => '280px primary, 200px secondary (30px padding)'
),
array(
'value' => '20',
'label' => '300px primary, 220px secondary (20px padding)'
)
after:
array(
'value' => '30',
'label' => '280px primary, 200px secondary (30px padding)'
),
array(
'value' => '20',
'label' => '300px primary, 220px secondary (20px padding)'
),
array(
'value' => '10',
'label' => 'custom (10px padding)'
)