There are a couple of reasons and potential solutions. The first way to avoid this is to weigh out your ingredients to make sure you are using the same amounts as in the recipe. There can be a huge variation in the way people scoop! Second is because there's also a lot of variation in the accuracy of oven temperatures. You only want to bake your cake layers until a toothpick into the center comes out free of crumbs. I usually start looking at cake layers 5 to 10 minutes before the cook time in a recipe, and I start checking with a toothpick every few minute or two once the cake stops wobbling. Moist cakes are so much better than dry cakes! Lastly, one solution for a dry cake layer is to soak it with a simple syrup. You can use my lavender simple syrup or create one of your own with equal amounts of water and sugar plus a flavoring, brought to a boil until the sugar dissolves, and then cooled slightly.