Difference between 2 graph?

[after310]

Hello

I have a small problem with graphs.
I'm graphing the activity of an interface of a switch with two graphs.

The first is generated "automatically" because a data query is associated with
the device (data query: SNMP - Interface Statistics).

I obtain the second graph with a graph template (created by myself) which uses OID.

These two graphs is about the same interface (if25 of my switch), the step and heartbeat are the same

Now, my problem: these 2 graphs are not exactly the same, the "automatic graph" show higher values (35MB) than "my" graph (28MB).

Is it that someone can help me find the explanation?

[after310]

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