What are you actually looking at? What is the voltage source?
I ask because it looks to me like there could be around 100mV of noise on that trace; the line is blurry, whereas on my Rigol it's pin sharp.
I would repeat this test using two or three 1.5V cells in series (because they are - effectively - electrically "silent"). Keep the wiring well away from noise sources such as nearby LED or fluorescent lighting, switched mode power supplies, etc.
Before you spend $$ on calibration, find out what your calibration lab offers. Many will put in a "standard" voltage (probably one for each range) and if the reading is within the scope spec they log the actual reading vs the spec and not make adjustments. Or if it is within but near the edge they will likely tweak the calibration.
Also consider you are likely using a 10:1 probe. How good is your probe?
You are best off taking Yoga to deal with your frustration 
The Rigol probe Input impedance is 1 MΩ.
The Technic VC88E input impedance is 20 MΩ.
That alone will give different results.
I am curious- what are you doing that you need laboratory precision?
Say what ?
Not if they are connected simultaneously, which I presume is the case here. Normally one uses 10:1 scope probes on the x10 setting (unless you like having restricted bandwidth and heavy loading) so the Rigol is probably 10M input impedance anyway.
Scope front-ends are very wide-band amplifiers and you have to trade off gain accuracy across the entire bandwidth - this is one reason they have larger tolerances than say a multimeter.
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