Help: Pipeline results dataframe has extra (hidden) entries in index

I'm obviously doing something wrong here (or expecting the wrong thing) but I can't see what it is (yes I've done the searching!)

The code below (condensed into a single cell for brevity) creates a pipeline which filters to the bottom 10 by AverageDollarVolume. I call run_pipeline() with a date range that spans 17 calendar days, 11 trading days. The resulting dataframe has 11 values in index.levels[0], as expected. Each day has 10 values in index.levels[1], as expected. results.info() reports that the multi index has 110 entries, as expected.

I want to extract the list of assets, so I use
results.index.levels[1].unique() which is a technique that is used in the documentation. However, this returns an array of 8930 (in my case) assets and this number is the same however wide or narrow I make the filter. I expected this to be the list of assets referenced in the index, so between 10 and a max of 110 (but probably more like 15), as run here. On one hand this feels like a pandas problem because run_pipeline() returns a pandas DataFrame and I then am only using pandas methods on it, but on the other hand it feels like a Quantopian problem because I have never seen this behaviour in a DataFrame produced by any other means. Help!

def make_pipeline(filterWidth=10):

    # Dollar volume factor  
    dollar_volume = AverageDollarVolume(inputs=[USEquityPricing.close, USEquityPricing.volume],  
                                        window_length=30)  
    # 10-day close price average  
    mean_10 = SimpleMovingAverage(inputs=[USEquityPricing.close], window_length=10)

    filter_dollar_volume = dollar_volume.bottom(filterWidth)  
    return Pipeline(  
        columns={  
            'meanclose': mean_10,  
            'dolvol': dollar_volume  
        },  
        screen=filter_dollar_volume  
    )

filterWidth = 10  
results = run_pipeline(make_pipeline(filterWidth), '2020-05-15', '2020-06-01')  
dateCount = len(results.index.levels[0])  
asset_list = list(results.index.levels[1].unique())  
print('''Number of dates in index.levels[0]: {0}  
Number of rows in dataframe: {1}  
Product of dates and filter width: {2}  
Length of results.index.levels[1].unique(): {3}  
'''.format(dateCount,
           len(results),  
           dateCount * filterWidth,  
           len(asset_list)))  
print('First 5 of asset list:\n', asset_list[:5])  
print('')  
results.info()

If you want to run it, my imports are these:

from quantopian.pipeline import Pipeline  
from quantopian.research import run_pipeline  
from quantopian.pipeline.data.builtin import USEquityPricing  
from quantopian.pipeline.factors import SimpleMovingAverage, AverageDollarVolume  
import pandas as pd

Thanks in anticipation...