Awe, I see, I missed the obvious difference, and thought you were talking about material variation with your past orders.
Some background first...
Seal script is almost the oldest form of Chinese (bronze script is and earlier pictographic form that led to seal script). In 221 B.C. the Qin Emperor came to power and set about standardizing the written language of his expanding empire. Up to that point, there were often 20 different ways to write each character (watch the movie "Hero", as this issue is a major plot point of that movie).
While a standard set of characters started to emerge, the variations of each character persisted somewhat.
Fast forward to more recent times...
Calligraphers often love and cherish character variations. If a piece of calligraphy has the same character repeated in the text, the calligrapher will try to find two different ways to write the character (thus avoiding exact repetition and maintaining artistic flow).
In the case of your artwork, Xing An-Ping decided to make the pieces artistic and educational, by showing two different ancient ways to write Wing Chun. Also, he avoided repetition even though they are separate pieces (I think he assumed they'd be hung in the same area).
You're probably already aware that Wing Chun can be written as
But how about the original seal script variations:
This is just a small sample of common variations. I have a dictionary of seal script variation that probably has 20 or more versions of each character.
The crazy thing about Xing An-Ping is, he seems to know all the variations of every character in his head. There's about 20,000 characters that you'll need to know to read ancient Chinese literature. He seems to know all of them, in 4 major styles (zhuan, li, kai, cao), and 4 more sub-styles (square zhuan, xing-kai, xing, xing-cao). Then he knows many variations of each character. There's something like 500,000 characters and variations in his head!
I can't compare this to English, except that you can spell things differently sometimes such as the name Sean, Shawn, Shaun. Or, the variation of handwriting from one person to the next.