Hi,
The reason it is different is that the base styles in SharePoint is different and therefore you must override other styles to get the layout you want.
It is hard to tell what the issue is without looking at it live, but if you use the developer tools to inspect (right click the item and select inspect) you can see HTML element and what style is applied. You can also modify the styles directly in the dev tools to test it out, but keep in mind that these changes are only temporary and is not saved.
In general when you move from SP2010 I think it might be better to remove all custom css and reapply it one step at the time because the approach will most likely be a bit different than it was in SP2010.
Alexander