1. We want attention : This is the most common one I've heard. People assume that when you dress in a plain modest style you want all eyes on you, this isn't the case. Rather most Christians are simply following this verse (not only with dress but with actions)
Do not be conformed to this world, but be transformed by the renewal of your mind, that by
what is good and acceptable and perfect.
As you can see here the Bible calls all women (and men) to be set apart for the Lord, shouldn't dressing modestly be apart of that as it says in 1 Timothy? Yes maybe we'll be given funny looks by people of this corrupt world but when the Bible is so clear on this point, does it matter? We aren't seeking the attention of people of this world rather we are looking for the attention and approval of God.
2. We believe this is a salvation point: Dressing modestly is NOT going to save you. It's the inside that should be pierced by the Lord before the outside is even considered. Modesty IS important because the inside should reflect the outside. But dressing modestly is NOT a salvation point.
3. We look down at others: another commonly heard misunderstanding people believe is that because we are dressed like this we believe our-self's to be better than other people. This is certainly NOT the case, we are ALL only human we ALL make mistakes, we ALL fail we all try to do better. WE dress modestly and plainly out of order form the Word of God. This doesn't mean we think immodestly dressed people are dirt and beneath us.
I hope this article has blessed you in some way, God bless!