The option you're seeing in Battery Saver only appears when the selected app has a Background Agent.
To be clear, no app can run in the background on Windows Phone. This is for battery saving and performance reasons. But apps can have a background agent which is a separate piece of functionality that runs separately from the main app itself.
So I was able to reproduce the issue you are describing using Microsoft's News app. Even when I disallowed background tasks in the battery saver it switched back to "Allow app to run in the Background" after using the app, specifically after going to the settings menu in the app.
The reason for this is that in the settings "Services" and "live tiles" are ...
The percentage shows the battery power a specific app is consuming. In the latest update, the usage in the background is shown seperately.
The usage data scales to actual device battery capacity, so full width means 100% capacity.
More background information on is available here:
Battery saver help page
Latest battery saver release notes
The "now until next charge" option effectively turns battery saver on early, until you plug in for charging.
All the normal battery saver things happen; so background processes don't happen, and email checks will be manual. This can be useful if you realise you've forgotten to take your charger on a weekend away, or you've been playing a few too many games,...
In order to turn on battery saver mode on your Nokia Lumia, go through the following steps:
Go to Settings.
Select battery saver.
Turn it ON.
But keep in mind that after turning it on you won't be able to receive WhatsApp, email and other notifications.
These apps show battery consumption because they do actually run on background and perform some work even though they don't have permissions to run in background. I can provide a few examples.
Outlook - Sync my mail in background
People hub - Sync my account, contacts in background
Messenger - Shows me push notifications
Loco Music player - Scans my music ...
It's pretty self-explanatory, really. If you enable that option, Battery Saver will be turned on until the next time you charge your phone, at which point it will turn off automatically. While Battery Saver is enabled, non-essential background processes will be disabled, as will be push notifications, meaning you won't receive notifications for new emails or ...