Cost index ranges for the various Boeing models:
737-3/4/500: 0-200
737-6/7/800: 0-500
747-400: 0-9999
757/767: 0-999 with Honeywell 200K and 700K FMC or 0-9999 with Honeywell non PIP, PIP and Pegasus Pre-2009 and Pegasus 2009.
777: 0-9999
Taking the 757 and 767 as examples. the original Honeywell 200K and 700 FMC's only had capacity of 200,000/400,0000 words and 400,000/1.4MB bytes capacity. This was insufficient memory capacity, to store CI values on a range of 0-9999.
When Honeywell's 1Meg/2meg 757/767 FMCS were introduced (originally Non-PIP FMCs which some airlines updated with the PIP software) with 1,000,000/2,000,000 words and 2MB/4MB bytes capacity the FMC now had sufficient memory capacity for CI values in a range of 0-9999.
The original Honeywell AIMS 1 on the 777-200/ER/300 had a 1,000,000 word and 2MB byte storage capacity. this being sufficient for storing CI values from 0-9999. This being the same capacity and available CI range of values on the Honeywell 747-400 FMC.
With the introduction of the 777-300ER and -200LR the Honeywell AIMS 2 upped memory capacity to 6,000,000 words and 12MB bytes.
The Honeywell 787 FMC has 15,000,000 words and 30MB byte storage capacity.
The Honeywell NextGen FMC, used on the 747-8 has 50,000,000 words capacity and 100MB bytes storage.
To sum it up, under 1,000,000 words and 2MB bytes storage capacity, the FMC could not store sufficient values to go beyond a CI range of 0-999.
The 737NG being the exception as Boeing never bothered to go above a CI range of 0-500.