question about arbitration scheme of AHB bus matrix

generate an AHB bus matrix ,using the perl script of CMSDK.

Below are the properties of the bus matrix:

• protocol:AHB2;
• arbitration scheme:fixed
• 2 masters:CPU,DMA(CPU is prior to DMA)
• 3 slaves:ROM(for instruction),SRAM(for data),sdram(other usage)
• conectivity:full

When using Modelsim to verify,I found that if i let DMA access ROM while CPU accessing ROM to fetch instruction,there will appear a phenomenon that  CPU work normmally,but the timing of DMA AHB interface make mistakes.

I suppose that because of higher priority ,CPU works without disturbing.In contrast,DMA AHB interface goes wrong.

What above is just my thought.I want to know why.