A new field will be set to whatever the default settings are. Its very easy for those default settings to be changed without realizing it. As long as nothing is selected anything you change will become the default. However if you select something and change the settings to make it the way you want that wont neccisarily become the default setting.
One way to avoid all this is to create a pallet layout and copy and paste similar objects from there changing only those things that need to change such as field reference or script calls.
Actually beyond that, what ever formatting properties the last selected object had is what will now be selected. So no matter what you set everything to, if you click on a purple checkbox to move it just a touch before you make that text field, then guess what...your new text field will now be purple too. Nature of the beast... =)