These different layouts are CRITICAL to the users' experiences on these OS's. The purpose for a unified UI approach is to make the user experience consistent from application to application. Once a user is familiar with where settings are for certain applications, they have a reasonable expectation that they will be located in a similar place for other apps. This greatly enhances the adoption of applications to users.
As an example, think of an 80 yr old person attempting to use an OS where there is no consistency between the apps. Each one would have to have an instruction manual and the user would never have a consistent experience. They would likely get frustrated and only use a limited number of apps if any. To put it yet another way, most runners who run long distances like to run on consistent turf. Its much more challenging and tiring to switch between track, concrete, asphalt, grass, dirt, etc. Don't make your grandparents do the equivalent with their phone...
Key point of post: When developing, follow the specific rules for OS's UI version.
NOTE: Even if the OS is the same, the UI experience can change from version to version. For instance, iOS6 and iOS7 had some big changes in its design guidelines. Make sure to plan for the new versions of the UI. Don't rest on your laurels...
When creating applications for multiple operating systems, follow best practices for the OS and version you are creating. Below are some UI guidelines for different mobile platforms.
Windows 8 UI guidelines - http://msdn.microsoft.com/en-us/library/windows/apps/hh465424.aspx
Apple iOS7 UI guidelines - https://developer.apple.com/library/ios/documentation/userexperience/conceptual/MobileHIG/index.html
Android UI guidelines - http://developer.android.com/design/get-started/ui-overview.html
Also very important to the overall designs, different OS's have different visions with how users will use their applications. For instance, Android has widgets and Windows have live tiles. Make sure to leverage these in your designs as bells and whistles, while just that, can make or break an app in the eyes of the user. If an app seems hacked and thrown together from a different design, the users can usually tell.
Remember, mobile apps are not just 'things to have' but a direct representation of a company's brand. Put your best foot forward or wait in the wings until you can. On the other hand, you can't make everything perfect as nothing ever is. It's a careful balancing act.