How much variability is present in SPR quantities?

I am doing simple preliminary experiments on the T200 to validate peptide binding to a receptor. I consistently obtain a KD that is ~4x weaker than the reported literature value (250 vs. 1000 nm). When evaluating my data, what should I look out for in determining if this is a significant difference?

This peptide was even custom ordered and has high purity so it is surprising there would be such a large difference. Are there any factors that I can look to vary? E.g. add more intermediate concentrations, flow rate, association/dissociation time, etc. I will say the curves show full association/dissociation at a flow rate of 30 uL/min

Kinetics between two components is dependent on the environment in where the kinetics is measured. In the case of SPR it ispredominately the buffer composition and temperature. When mass-transport and rebinding can be ruled out, heterogeneity either from the ligand (immobilization) or analyte is minimal and full association/dissociation curves are available I would say look at the buffers. Literature values are notorious incomplete in reporting k_a and k_d and the exact measurement conditions (buffer components, pH and temperature).
However, when the experimental conditions are the same a good agreement is possible as is shown in the benchmark publications below.

Bravman, T., V. Bronner, K. Lavie, et al. Exploring "one-shot" kinetics and small molecule analysis using the ProteOn XPR36 array biosensor. Analytical Biochemistry 358: 281-288; (2006).
Rich, R.L. et al. A global benchmark study using affinity-based biosensors Analytical Biochemistry 361; 194-216; (2008)

Kind regards
Arnoud