Why change in total kinetic energy is same for Two different systems ?

The question is from Physics book titled "Physics for Scientists and Engineers" by Randall D. Knight.  Attached is part of chapter 10.1.

"Objects A and B are oblivious to our choice of system, so change in total kinetic energy for system 2 is exactly the same as for system 1."

How is that possible?  System 1 has kinetic energy because there are external forces but System 2 doesn't have external forces.
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naseeamAsked:
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David Johnson, CD, MVPOwnerCommented:
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.[1] It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body when decelerating from its current speed to a state of rest.
https://en.wikipedia.org/wiki/Kinetic_energy
System is not a physical object 2018-09-07_16-28-43.png
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naseeamAuthor Commented:
Is System 2 isolated system ?
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David Johnson, CD, MVPOwnerCommented:
yes system 2 is an isolated system perhaps only gravitational forces are in play
System 1 has no relation to System 2
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