Hello Diego,
You're actually not showing the same screen, but indeed the experience is a bit difference between legacy web client and Unified Interface.
On your first screenshot, you are on the account form, on the legacy web client, and you are showing the activities that are available from the account form ribbon.
On the second screenshot, what you are showing are Quick Create options that are available regardless of where you are in your app (so regardless of the account, in your this case).
In the Unified Interface, you can add custom activities on an Account form using the timeline, or by navigating to the "Related" / "Activities" part of the form.
Henry
