Spectrophotometers are not created equally - they all have different bulbs, bulb ages, and detectors. While the data from one spectrophotometer are not directly comparable with another spectrophotometer, the trends are (e.g., OD over time from each instrument should give a similar profile). In short, you would never expect to get the same value from on spectrophotometer to another, thus you must always use the same instrument when making measurements to eliminate this variation.