Nobody knows exactly why some people are affected by these rashes and others with the same exposure aren't. On the other hand, those with contact dermatitis do have true skin allergies, but usually not other allergic problems like someone with atopic dermatitis. Contact dermatitis occurs at all ages, whereas atopic dermatitis is most often a problem in kids. Hand dermatitis is caused by skin irritants such as detergents or chemicals. If your skin surface doesn't have enough oil, there's more evaporation of water which contributes to the dryness.
Scratching can actually trigger eczematous rashes.
Initial mild itching makes you want to scratch, of course. But scratching damages the skin, worsens inflammation, irritates surface nerves, and leads to even more intense itching than you had before. This cycle aggravates and prolongs all of these rashes. In fact some eczematous rashes completely disappear without any treatment if you just stop scratching. As you can tell, these rashes differ in many ways.