Knighthood was more than a social rank. It also had religious significance. The buffet or slap a warrior received at his knighting ceremony survived until the 20th Century as the slap a Catholic bishop gave to each youngster at Confirmation to brace them for the rigors of the Christian life.
Even the rampant lion of Flanders had Christian significance. During the Crusades the lion symbolized a Christian knight, and lions adorned the flags of many Christian kingdoms.
From this rich symbolic vocabulary, then, it seemed natural to choose a secondary charge with religious significance. The three crosses represent Calvary, where Jesus was crucified between two thieves. The implication for someone steeped in Christian culture is that I may be a sinner, but I want to be like the repentant thief, not the scornful one.
The green background is just because I like green.