I’ve noticed some Mango items fit differently even though I order the same size, how can I be sure about the size?

The sizes of clothes for women aren't always the same. A lot of this is because of something called "vanity sizing." The idea is that women are more likely to buy a dress that says "size 8" than one that says "size 12."
Mango sizing tends to vary between different items, too. While some pieces may fit true to size, others can be either smaller or larger depending on the fabric, style, or cut. So, how do you go about getting clothes? Don't pay attention to the labels; instead, look at the store's size charts online. To find something that fits, you may need to go try on a lot of different sizes in person.

These are what we can suggest, if you know more in terms of experience, please share in the comments.